IT'S A PRETTY GUTSY thing, being so sure of that proportion and shape and style of clothes that suit you best that you can confidently pass up current styles.

But there is a stubborn minority of men an women out there who have chosen not to choose. The have found their "uniform," which they tend to wear regularly for their daytime routine.

Jacqueline Onassis wears a uniform. And in our town, so do Deeda Blair, member of the President's Task Force on Breast Cancer Research of the National Cancer Institute, and wife of attorney William McCormick Blair, former ambassador to the Philippines and Denmark; Bunny Mellon, wife of National Gallery president and East Building donor Paul Mellon; and Susan Mary Alsop, author and former wife of Joe Alsop. Elsewhere, Hubert de Givenchy has worn the same style white linen jacket, supposedly designed for him by Balenciaga, for more than 20 years. And Halston is always in a turtleneck and trousers, usually black.

Hereabouts, designer Jeffrey Bigelow wears, day in and day out, day and evening, an open collar white cotton shirt with blue wool gabardine pants.

It's hardly that these people cannot afford to stock lots of different things in their closets, But they believe as fashion [WORD ILLEGIBLE] Diana Vreeland says, "Variety (in clothes) is absurd, particularly for going to work."

It certainly makes for a no-hands start to the day when you can eliminate the question of what to wear. And while it sounds like the all-time money saver, these uniforms are often the top styling (and price) around, frequently Custom-made, always best quality.

Several uniform wearers admit that they will occasionally dip into the new styles - which invariably end up pushed to the back of the closet.

But when they do break out is a totally different kind of dress, say for evening, it adds to the special quality of that occasion. Deeda Blair and Diana Vreehand both say that for evening their clothes are "pure fantasy," a bold contrast to the tailored styles they like for day.

It's not a new idea at all. Alice Roosevelt Longworth understood well her own best style and had Madame Agasta create many dresses in the same form, a loose chemise style worn quite long, sometimes belted, that she topped with a huge brimmed hat. Rather than bowing to the sytles of the moment she developed her own style, her unique form, her uniform.

Confining one's daily dress to a small group of quality items has been a formula for European women. More typically Americans tend to have a vast array of styles in their closets. It never seems to disturb an elegant French woman to wear the same clothes, with perhaps a small change in accessories, many days running; she hangs them out to air them and spot cleans them to keep those items in top condition for years. Many French women, both wealthy and not, limit themselves to a clothing regimen, switching around a few very good pieces of sportswear, adding occasionally to the assortment with, say, a new silk shirt.

And many designers, Saint Laurent among them, keep that style of dressing in mind when they choose colors each season, so there are things their followers can purchase that combine well with things bought in earlier collections.

"I wear the great things of the 20th century," says Diana Vreeland, who was editior of Vogue and Harpers [WORD ILLEGIBLE] and is new consultant to costume Institute of the Metropolitan Museum of Arts in New York. She's referring to sportswear items of course, sweaters, skirts, shirts and pants, never venturing for the daytime very far afield in color or style.

Her turtleneck sweaters are of her own styles, not the "tight goosemack variety," she says, but cut looser, a la Balenciaga. "I used to cut them on the floor of my living room to get the kind I like," says Vreeland, who now ha six in each mere form Braemar that fill the bill.

Pants are always from Mila Schoen - "There's no tailor like her in the world, and no fabric like it," says Vreeland of the Italian designer's things. "The hardest problem was finding absolutely the perfect shape for the shirt." But she has, now she rarely parts from it."

I'm not much tempted to wear anything else for work," says Vreeland, whose pulled-back, pitch-black hair and rouged cheeks are, in a way, part of her uniform.

Deeda's Blair's "uniform" is a Jean Halm shirt dress in a crepe de chine-like fabric in a mauve print that she has been wearing about four days a week all summer long. And she always rolls up the sleeves, carries the same Irish basket, but changes shoes. "It's marvelous for going to meetings, then with opening one button, going directly to dinner," says Blair.

The dress was actually a gift from a friend, Jane Wrightsman, who shares her taste in clothes. When they find a great item, each buys it for the other. Before she got the Halm dress, which Wrightsman also owns, Blair bought her friend a pair of gray gabardine pants. "I clearly got the best deal," says Blair laughing.

Even when Jacqueline Kennedy Onassis was first lady, and was spending a reported $30,000-plus a year on clothes, there were clear signs that she well understood her own best style. Styles were kept simple and ungimmicky, but with no limit no fabric and color. Her dresses and coats were often bright and bold, her favorite fur, attention-getting leopard.

But she was faithful to certain accessories that worked well for her . . . large sunglasses, square silk scarves, the pillbox hat, low-healed shoes.

In recent years she has tamed her bouffant hairstyle, appears much slimmer and seems to have put reins on the kind of clothes she likes to wear for day.

Making selections at Halston, Calvin Klein, Geoffrey Beene and off the import racks for her daytime garb, she appears to be partial to a shirt or sweater and pants uniform - during the winter in navy and black, in the summer, in white. Recent pictures have shown her in softer blouses and skirts as well. Coats never get much trickier than a simple wrap or trenchcoat.

For evening, it's fantasy time, with Mary McFadden among has favorites, and a white pleated tunic style dress obviously a favorite - she has worn it both in New York and Washington.

Blair, who is a member of the Best Dressed Hall of Fame, isn't sure about when she discovered her "uniform." She says she just gravitated to the proportion that is best for her and has stuck with it. Her last winter's "uniform" was on Yves Saint Laurent costume of a cashmere skirt, wool blouse, caps and shawl, mostly in shades of blue, which she always wore with dark red boots. Occassionally she would change the shawl and for evening would add beads.

"When you pay so much for an outfit you feel you should wear it all the time," says Blair, adding that she expects to be seen in it a lot this fall, too.

Eve Lloyd Thompson calls her 7-year-old Ralph Lauren blazer her security blanket. "If there ever was a fire out here," says Thompson, who raises beef cattle and thoroughbred horses in Poolesville where she lives with her dentist husband, "I'd race for my custom boots, riding jacket and Ralph Lauren blazer."

Not everyone admires or understands "uniforms." When designer/manufacturer Jeffrey Bigelow showed up the other day to meet banker friend Richard Bernstein, Bernstein looked at him quizzically. "Don't you ever wear anything else?" he wanted to know.

It was a fair question. Bigelow always wears white cotton custom shirts, with no pockets and a vertical monogram, from the Custom Shop and wool gabardine trousers from Madonna in New York. In the winter he adds a tie and jacket "only because it's cold," he says.

Bigelow locked himself into the open shirt and trousers look about three years ago. "I couldn't find a decent pair of slacks in this city. If the fabric was good, the construction was lousy, or the shape ghastly," he recalls. The shirts weren't much better in this town, he says. Heavy in bad colors and a shortage of all-cotton.

On one of many New York business trips he discovered the boutique Madonna had beautiful, lined wool gabardine slacks, a good shape - not too wide or narrow. "They are $120 a shot," says Bigelow, who now has collected five pairs, "but they look like it and I wear them all the time."

He had bought expensive shirts - $33 to $40 in Washinton stores, but "they never fit the way I liked them and they weren't always all-cotton." He tried custom-made shirts in Washington, but soon found a man at The Custom Shop in New York who fit them better.

"I now pay $28 for a shirt that fits the way I like it, sleeves are perfect, the tall is long enough, the body is snug enough," he said. He now owns about a dozen white and a dozen in colors. And wonders why everyone doesn't do this.

"Once I found my shirt and my trousers, I quickly realised I didn't need to find anything else." He admits that occasionally since, he has bought other things, "but I'm always disappointed." The exception, Lacoste knit shirts and a Ralph Lauren hacking jacket. Slip-on shoes by Moreschi from The Brass Boot complete his uniform.

"I've seen a lot of people look super in suits, and other clothes, but I know I'd look like a hero in them," says Bigelow.

Bigelow adds, "Maybe it is a cop-out. It's a routine, maybe even a rut, but I like it. It's the best quality I can find and I can depend on it."

Former ambassador Trae Davis picked up his first embroidered shirt - he wears them during the summer as part of his evening costume - in Cuba, in pro-Castro days, and has built a wardrobe with 20 or 30 of them, buying them wherever he sees them on travels, shops, mail-order whatever, always to wear with white trousers.

The big appeal, says Davis, is that it is the most comfortable thing to wear for summer - in fact he has developed a version of winter, too. "It's European, international, Mediterranean, maybe," says Davis, "showing lots of skin and hair. And the complete opposite of my conservative look for day."

During the winter Charlotte Fleming, teacher and model, confines herself mostly to a few good turtleneck sweaters and well-cut gabardine pants. Summer's another matter. In the heat she wants to pare down to minimal clothes. "If I had my way, I guess I'd be in a bathing suit all the time." In fact, she swims a lot, and is in a bathing suit a lot of the time.

The nearest thing to a bathing suit she can find is a strapless cotton dress bought three years ago in Montgomary Ward in San Diego for $8 on sale. She was wearing it the other day with $89 Charles Jourdan shoes, and a $39 African belt. "I live in it."

"It is most important for a woman to find her own form, her unique form," said French designer Sonia Rykiel at a recent lecture at the Smithsonian Institution."It is not uniformity. It is the place of a woman. Like her house, her bed, the street where she lives, the shop where she buys the food she eats. It's a mixture of all this. And it is a unique place where she is completely herself," she said.

But for the several women who asked Rykiel how they discover their "uniform," Rykiel brushed back her long red hair, threw up her hands and said, "I can't help. You must find it alone. She must try many things, look in the mirror. And keep doing it till you find your unique form.