If the genie in the bottle gave me one wish, here's what it would be: That Congress could not pass one more tax law unless each and every senator and representative agreed to fill in their own returns. Then, and only then, would we get true simplicity.

At the end of December, still more tax changes came down the pike. A few will affect the returns you're working on now:

If you underestimated your 1987 tax, you will pay no penalty if, during calendar-year 1987, you paid at least 80 percent of what's due (or 100 percent of your tax for 1986). Had the new law not been passed, you'd have been in the dog house unless your estimated tax reached at least 90 percent of the taxes due.

In late December, the IRS announced a blanket waiver, for the 1987 tax year only, of penalties for underpayment caused by underwithholding of tax from wages. This waiver applies to income from wages or salary only.

But the change doesn't abolish the 90 percent rule. It merely defers it into 1988, so this is the year to get squared away on your tax withholding and estimated tax payments. If you bought a publicly traded limited partnership, you might have planned to shelter its income from taxes with losses being generated by your other partnerships. Tough luck. Congress changed its mind and made the new rule retroactive to 1987. You can now use your partnership losses only to offset gains from the same investment.

If you do business as a partnership, personal-service corporation or S corporation, 1987 was supposed to be the year to end fiscal years (which don't coincide with calendar years). Tax planners mourned because fiscal years allow you to push income into the future instead of paying taxes on it now.

At the last minute, fiscal years were restored. But if you use them, partnerships and S corps have to pay the government a deposit, based on what the tax would have been had you chosen a calendar year.

So much for the news that you need for completing your current income-tax returns. Now for the rules affecting tax-planning in 1988:

If you're a homeowner: Did you take out a mortgage or home-equity loan after Oct. 13, 1987? Starting with your 1988 returns, new caps have been placed on the amount of home-mortgage interest you can deduct. Some will find the caps more generous than the old '87 rules, others will find them tighter.

You can deduct interest on any loan up to $1 million that is used to buy, build or substantially improve your primary home and one vacation home that is used as a second residence. (The $1 million limit applies to both residences together.)

You can deduct interest on a second mortgage or home-equity loan of as much as $100,000, regardless of what the money is used for. There's no deduction for higher amounts.

You can refinance your house for the same amount as the existing loan balance and still keep your tax deduction, says Bill Kelliher, a partner in the Washington tax office of Peat Marwick Main. But what if you take extra money from a refinancing? It's deductible if you use it for home-improvements. It's also deductible if it can slip under the $100,000 cap available for second mortgages, according to an IRS spokesman.

For working couples: The cost of sending your child to an overnight camp will no longer count toward your deductible child-care expenses. Congress is happy to help pay for baby sitters, but not if they're wearing Girl Scout shorts.

For spouses and children (age 18 and older) working in an unincorporated family business: Formerly, you could skip the Social Security tax. Now, you have to pay it (7.51 percent this year, from both employer and employee).

Social Security taxes have also been extended to farmhands, military reservists doing weekend duty, and earnings from tips that are reported to the IRS.

For people doing business as personal-service corporations, such as doctors, dentists, architects, accountants and certain consultants: You've lost the graduated income tax on corporate profits, which taxes some of your business income at a lower rate than your personal income.

If you're part of a family business: Congress ended the old trick of freezing the value of the business at today's prices (to reduce dad's estate taxes), while letting him run the business and pass the appreciation to his kids tax free.

To get this tax break, dad now has to retire. Could it be that the kids had a lobbyist in Washington, working at getting their interfering father out the door?