PERHAPS because her bite is worse than her admittedly formidable bark, Miss Manners is in sympathy with people who wear braces on their teeth. However, the sympathy ends abruptly the minute they reach into their mouths at the dinner table to extract construction materials.

Head braces with dried dribble on them, which look as if they had done years of duty in the mouths of cab horses; tiny round rubber bands, left around in pairs like the tooth marks vampires; retainers cunningly made of pink plastic to resemble disembodied palates -- Miss Manners is not oblivious to the suffering associated with these, but does not think they do much, when left lying around, for the smiles of others.

But the etiquette of braces begins before this equipment is issued. It begins with knowing that when the first child in a class to get braces appears, you do not make remarks about train tracks, Jaws or Metal Mouth. That child is likely to be the first person out of braces, too, when his tormentors are themselves locked firmly in.

Braces can be, however, a subject of polite conversation at the appropriate age levels. "How long have you had them?" and "How long do you have to have them?" are as standard and inoffensive conversation openers among the braced as the weather, the economy and the general decline of civilization are among their presumably straightened-out elders.

It is also permissible to discuss the reason for your braces -- underbite, buck teeth, gaps. This is a bit more personal, but has become unremarkable, in the way that naming the causes of marriage break-ups is among adults.

But the observation, "You're not supposed to eat that" has the same unacceptability as the same phrase does to overweight adults. Only the most privileged relationship (parent to minor child) justifies this. Everyone else is expected just to watch as someone with braces bites into a caramel: That person has already heard the theory from his dentist and will undoubtedly discover the truth of it for himself after this little experiment.

For authorized eating, the rule is to do whatever is necessary to enable others to enjoy their meals. This means that rubber bands, head gear and retainers are removed in privacy before one appears at the table and reinstalled in privacy afterward. In the meantime, they are kept in their cases, not parked next to someone's dinner plate.

The chief problem, then, is transferring the food from the plate to the digestive tract. When one has braces, it tends to make a stop-over on the front teeth.

Any cleaning up that can be done by the tongue with the mouth closed is legitimate. A quick scrape with the thumbnail in the mouth isn't legitimate, but if Miss Manners doesn't see you do it (the napkin goes up to the mouth with four fingers visibly holding it, so that it appears the lips are merely being wiped), it doesn't count against you. Serious picking with tools must be conducted away from the table. This is a nuisance, but it does give one the pleasure of having a second helping when everybody else's meal is over.

The most important rule is that social embarrassment over the fact of the braces is not permitted. House guests should bring their head gear and their hosts are honor-bound not to notice that the guests have drooled on the pillows. And refusing to smile for 2 1/2 years does nothing at all for one's social standing. MISS MANNERS RESPONDS

Q. How do you suggest I handle myself when my husband and I are in company and start to have a disagreement? Should we excuse ourselves to another room, or put it on hold until we get home? My husband always wants to discuss it right there in front of everyone, when he does, I become uncomfortable.

A. If you go into another room and hold an argument, you will make it difficult for the other guests to hear what is going on while pretending to be making small talk among themselves.

If you stage it in front of them, it seems only fair to let them participate. Perhaps you could divide everyone into teams, like charades, or just ask afterward for a vote on the merits of the case.

Would you mind telling Miss Manners what sort of an argument develops at a party and becomes so explosive that it cannot wait until you go home? Undoubtedly, it begins with one of you critiquing the other's behavior at the party -- in itself a habit that it both rude and maritally disasterous.

But that is your private life; Miss Manners only asks that you keep it private. The entertainment value to others of your spats is probably a great deal less than your husband seems to believe.

Q. Some help for St. Patrick's Day -- is it mannerly to have more than three Irish coffees for breakfast? If one has no green to wear, what next? Is it true one must not jog, but jig on March 17? If one cannot afford green carnations, will candy be okay for an overweight girlfriend?

A. Let us put it this way: There is nothing inherently rude about having more than three Irish coffees for breakfast. It may even solve your problem of wearing something green, as could jigging in the resulting condition or looking at candy. Whether Irish coffee consumed in vast quantities will lead to rude behavior is another question.

Feeling incorrect? Address your etiquette questions (in black or blue-black ink on white writing paper) to Miss Manners, in care of this newspaper.

Copyright (c) 1983, United Feature Syndicate Inc.