While you can make a case that winter is hazardous to your health, maybe you'd better not try to list a trip to Tahiti as a deductible medical expense. Avoiding the flu by seeing Tahiti, says the Internal Revenue Service, definitely won't wash.

On the other hand, as the deadline for income taxes closes in, it's not a bad idea to find out what kind of travel expense does qualify for a deduction. For Instance:

Good deeds: Sometimes virtue is rewarded. If your travel benefits an approved, U.S.-based organization operated for the public good, you may be able to deduct your expenses.

What that means is that you usually can write off the cost of a trip to Africa of Asia if you go as, say, a volunteer teaching English at a church mission. It's also likely to be okay if you put in your time on a three-week work project at an Indian reservation in the Canadian Rockies.

In fact, the list of possibilities is quite lengthy -- and if your do-gooder instincts are aroused, there's an easy way to get a line on upcoming opportunities. Just out is the 1981 edition of "Invest Yourself," which details more than 100 community-development projects, work camps and institutional service programs both in the United States and abroad. It's published by the Commission on Voluntary Service and Action, and you get it by sending $3 to "Invest Yourself," c/o Susan Angus, P.O. Box 117, New York, N.Y. 10009.

Your deduction hinges on how much aid you give, i.e., was your volunteer work the principal reason for the trip, did it take up the normal amount of work time, and did you come straight home as opposed to dragging the trip out twice as long for personal pleasure? You can't include anything for the value of your time or service but in general you can deduct food, lodging, transportation, tips and incidental maintenance items like laundry, both en route and at the destination.

Improvement trips: Forget about writing off the costs of sending your honor student on a study tour. You, on the other hand, may be eligible for a deduction if your trip was to gain knowledge that better equips you for your present job or general line of work. Example: A hairdresser who takes off for a month in Paris might well qualify if all the workdays of the month were spent taking a coiffeur course.

However, an amateur cook who travels to attend a cooking school that will help turn him or her into a pro had better get a restaurant or catering job first, because Uncle Sam won't underwrite anything not directly connected with what you already do for a living.

All those taxes attached to trips: If you use the short form, or if you itemize but take the standard amount for sales tax, the fact that sales taxes have soared with inflation may be meaningless. But if you do a lot of high-bracket personal travel in this country, it may pay you to have a closer look. With $100- and $200-a-night hotel rooms and $50 dinners, sales tax can, in short order, add up to something stiff.

Not all sales taxes are deductible, though, and remember, too, that not all hotel taxes are sales taxes.

Unfortunately, foreign sales taxes (such as Britain's 15 percent), foreign airport and departure taxes, and U.S. excise taxes (which include customs and airport taxes) are not allowable as additions to your total. Nor are any service charges.

Tax-deductible trips: If someone tries to sell you a charity-connected tour by telling you it's deductible, you might consider smacking him sharply on the knuckles. He is overstating the case. There are many tours sponsored by tax-exempt organizations, but the gimmick is that only the amount (usually small) that goes directly to the charity, institution or foundation is deductible. It falls under the heading of "contribution." And while such tours may even have educational aspects, the payoff as a rule is strictly in pleasure, not in total trip exemptions.

Business expenses made expressly to meet job requirements, of course, are as deductible as ever, though naturally subject to scrutiny. Lavish living is reportedly suspect, but you'd never know if the way hotels are expanding and emphasizing luxury extras.

Conventions: Well, the IRS has just revised the tax-deductibility rules on these, and it's basically back to the old yardstick -- if they can meet other business-deduction rules, anything goes except cruises. To determine the "reasonableness" of a meeting, the IRS looks at four things: the purpose of the get-together and the activities taking place; the purposes and activities of the organization; where the active members live, and where other meetings have been or will be held. As of now, though, the new rules eliminate old special-reporting rules and per-diem limits.

Oh, yes: If you own rental property elsewhere, the IRS people understand and allow deductions for visits to go see it and fix it up. In fact, they don't blink unless something looks funny. For instance, spending $10,000 on travel to a property that shows a profit of $2,000 would probably trigger their desire to see you in person to hear your explanation.

Little bits. If you buy medical insurance to cover sickness or emergencies while traveling, you can deduct its cost as a medical expense. You can knock off for inoculations in the same way.

If you're big on visiting churches and museums while traveling and, instead of admission fees, the churches and museums ask for donations, write a note to yourself; that's a deductible contribution.

You can also have a few dinners on Uncle. How? Well, if in the course of a pleasure trip you take time out to conduct some business and do so at the Ne Plus Ultra Restaurant, it's as allowable as it would be at any other time. A well-kept log listing dates, places, guests and purposes is useful evidence here. But whether you can bring a spouse or companion along and still have the evening count as deductible remains one of those hard calls.

Then again, if you couldn't count on the IRS to leave you swinging in the wind, some people would get greedy. Not you, of course.