"This time, like all times, is a very good one if we but know what to do with it," runs the Emersonian motto of Changing Times magazine, and last month the 32-year-old publication decided it knew what to do.

It is accepting ads for the time in its history, ending the most notable holdout since Reder's Digest started taking ads in 1955.

If you're going to do an about-face you might as well do it big, and certainly nobody over at Changing Times is apologizing for anything.

Beginning with the March issue Changing Times will be accepting "any ads that are tasteful and legitimate," said Nicholas H. Nilese the publisher, in New York. "To get directly at your question, that means tobacco and liquor. n We're doing it because it's the most sound economic decision."

With costs rising around them like the tide around a sand fort -- "second class postal rates are up 700 percent since '60" -- they had two choices he said: raise subscription and newstand rates or go to ads. But the price, $12 a year, has been a point of pride for years and, remarably loyal as the 1.4 million readers have always been, there was some question of their standing for a price rise.

"Our readers are very money-conscious," remarked Austin H. Kipling, who founded the magazine with his father, in 1947.

"They've very sensitive to price. In the '74 recession we noticed how sensitive the magazine was, even more than the newsletters. We like to think we're indispensable, who doesn't, but I'm afraid that when you come down t it, only the daily newspaper achieves anything like the status of a utility."

At that, the Washington newsletter -- the most widely circulated letter in the world -- has become something of an institution since it was launched in 1923. W. M. Kiplinger, an old newspaper hand and the Washington correspondent for a New York bank in the early '20s, founded it on an impulse. He was running a query service for financial clients in New York and decided, one September evening, to answer all the questions at once in a letter summarizing the Washington business news of the week. The Kiplinger Washington Letter has been coming out weekly ever since.

There are others now in the same concise format: the Tax Letter, Agricultural Letter, Florida Letter and California Letter. All give briefings on new government programs, policies, politics, market trends, taxes, industrial plans and techniques, income and price outlooks, in a word, "anything that will have an effect on you, your job, your personal finances in the future."

The senior Kiplinger developed the snappy style that is a hallmark of his publications: colloquial, direct, underscored with key words and phrases. He also introduced the systematic use of comment instead of quotes. Sources are never quoted by name, which means he can say much more than conventional news media.

The Washington Letter gave details of the Marshall Plan in 1949, four months before it was announced.

"There were a lot of eyebrows lifted when my father started doing this," said Austin Kiplinger. (His father died in 1967 at age 76.) "But now it's pretty standard. We're not in the news business, really. We're talking about judgment. We're talking about the future. When we deal with the past, it's in terms of projections."

Many a senator who must publicly insist that the bill he sponsors will pass this year can privately send word to Kiplinger that he won't make it this time. So Kiplinger is able to predict the bill's failure.

The long-range predictions tend to be more accurate than attempts to guess what will happen in the next month or so, Kiplinger said. A July 1979 article on the gold boom saw prices going as high at $300 by year-end, but no farther. But who could have foreseen the panic buying that shot the price half again higher?

Generally, the Kiplinger batting average is good. And surely one important reason is that this is a venture in true group journalism. The full-time staff of 16 editorial and management people on the magazine, plus about the same number for the newsletters, work together so closely that they usually have little trouble reaching a consensus on the interpretation of events.

"One big difference we had was over Nixon, whether, and later when, he would quit. Some of us had him leaving office a year earlier than he did, but I felt he'd stay on longer. They were looking at the evidence of Watergate. i I was looking at the personality of the man."

It is this sort of Washington savvy that gives Kiplinger a special position among our political and economic soothsayers. Very much a part of the capital scene, he serves on the board of the National Symphony and the Washington Journalism Center, among other things. He also keeps in touch with local society figures at the Potomac Hunt and Chevy Chase clubs, not to mention the Metropolitan Club and a long string of professional and educational associations.

His contacts in these different worlds and in youth welfare activities give him an invaluable bank of authorities, nearly always unnamed in the magazine, on the broad variety of subjects that Changing Times touches.

(Topics in a recent issue: investing during a recession; facts on gasohol; medical self-testing kits; the roller-skating fad; no-fault car insurance; how a gas meter works; tips on car batteries, wines, scuba diving, passports and vending machines. And, as they say, much, much more.)

Starting with informal conferences on Monday, the staff puts together a mix of articles during the week, subjects the whole thing to free-for-all criticism, and somehow every Friday comes up with a revision that everyone can live with.

"Sometimes," Kiplinger mused, "I shudder at the prospect of something I've just written. On the basis of the increasing importance of primaries in electig presidents, we've come up with the notion that Reagan is looking good, very good. Whew!" He rolled his eyes.

Though the letters are for business people and magazine for consumers, they are considered the same audience, since business people are apt to be the most concerned consumpers of all. Each issue covers developments in the basic areas: housing, transportation, clothing, savings and investment, insurance and jobs. Then there are the "shorts," everything from traveling with pets to "Babies Know More Than We Think.

Less involved in the nuts-and-bolts approach of Consumer Reports, with its product tests and brand-name evaluatin, Changing Times takes a broad view of consumers.

"Right now we're interested in maintenance and repairs," Kiplinger said. "People are paying more attention to that. Our replacement economy has got to change. So we'll be writing about that."

He doesn't expect the editorial content to change with the new ad policy. Some staff has been added, and editorial volume will rise 13 percent by March. Already some layout shifts are apparent, and some pages are running in three-column form. The typeface may change. And sales operations will open in Chicago and Los Angeles.

"But we don't expect to see a net gain before 1981. I don't think we'll lose any credibility just because we have ads. People are accustomed to seeing ads. We've never been highly lucrative, but we have survived, and this change will let us carry on. Our subscription rate has taken 32 years to go from $6 to $12, and we don't want it to go any farther."