It is hard to recommend to the average taxpayer that he fill in his own income-tax return. Tax law is so complicated, and so overlaid with exemptions and exceptions, that only the truly dedicated can make their way through the instruction book. Only the truly lucky will get things absolutely right.

Very simple tax returns still yield to do-it-yourselfers. If all your income is from wages, tips, dividends and savings-account interest, if you have not complicated your life by owning a house or getting a divorce, and if you use the standard deduction, your income taxes should be an easy job. The simple, short form 1040A is orderly, and its instructions reasonably clear.

Even so, it is not always clear who should use the short form. For example, if you meet all the conditions listed above--and you also have a child in day care while you work--using the short form will cost you money. You are entitled to a child-care credit worth up to $400 on your 1981 return ($800 for two children or more). But that credit can be claimed only on the long form, 1040.

Many people are accustomed to getting free tax help from the local offices of the Internal Revenue Service. But budget cuts have eliminated the staff that used to sit down with you individually and help fill in your return. IRS staffers are now available only to answer particular questions. The thousands of taxpayers who crowd in for help around April 1 will have to crowd into the offices of commercial tax preparers instead.

The big tax chains charge an average of $32 to fill in federal, state and local returns. (The highest fees are generally in New York City; the lowest, in Texas.) Certified public accountants charge much more, and are the only sensible choice for people with higher incomes and a profusion of deductions. But for a majority of taxpayers, the big chains do an adequate job.

Larry Johnson, director of marketing at H & R Block, says that his typical client is a middle-income working couple who own a home and file the long form. J.B. Boyd, director of the Beneficial Tax Service, describes his average client as a blue-collar worker between 45 and 50 who is married, with two children, owns a home, and earns $20,000 to $25,000.

Every tax season, one or another consumer advocate takes a standard tax return to several tax preparers and publishes varying conclusions. The error rate increases with the complexity of the return. One preparer may say that the hypothetical taxpayer owes several hundred dollars in additional taxes; another may say that he is due a refund.

This annual exercise doesn't prove that tax preparers are dummies. It shows only that the tax code is ridiculously difficult. If the same return were given to a sample group of individual taxpayers, their error rate would doubtless be much higher than that of the professionals.

If a tax preparer says that you owe a large income-tax bill, it wouldn't hurt to get a second opinion. But published error rates should not discourage you from seeking outside help.

You cannot walk into a tax preparer's office and expect him or her to divine all your deductions. The preparer needs time to question you about your financial affairs. The closer you get to April 15, the less time the preparer can give you and the more apt you are to overlook deductions.

Before going to the tax preparer, read over the instruction booklet that came with your tax return. It will tell you where to look for deductions and tax credits, and give you an idea of what financial documents to take to your tax session. At a minimum, take all records of your income, check books, investment records and the year-end statements that come from banks and mutual funds.

Try to sum up deductible expenses like interest payments, alimony and work-related moving expenses. The more organized you are, the better the preparer's chance of finding deductions you hadn't thought of.

On the whole, it pays to deal with the big chains rather than with services that open in February and close in May. The chain should still be around if the IRS questions your return, and should send someone to the audit with you to explain how the return was prepared. If the preparer made a mistake, the firm generally pays any interest or penalties due (always ask about this in advance). But you pay the additional tax.