So the dreaded tax-filing deadline’s come and gone. What’s next?

The impulse is to shove the whole matter aside and lick your wounds if you had to write a check or happily await your refund.

But there’s a chore you should do first — adjust your paycheck withholding so that you’ll pay the right amount this year. That’s done by filling out a new W-4 form, available from your payroll people at work. All IRS forms also are downloadable from www.irs.gov.

If you had to pay more tax after completing your return, you had too little withheld from your paycheck and may well have paid a penalty on top of the tax due. In that case, have the withholding increased.

Getting a refund means you had too much withheld and gave Uncle Sam an interest-free loan. Better to have received that money in your paycheck and put it to work in savings or investments. Reduce your withholding.

This assumes, of course, that your tax situation will be about the same in 2005 as it was in 2004.

Often, it’s investment income, rather than incorrect withholding, that causes a tax-due bill when a return is complete.

If you sell profitable investments or receive sizable interest or dividend income during the year, you can avoid penalties by paying estimated federal taxes every quarter. Use Form 1040-ES. Check also on state estimated taxes.

One of my recent columns looked at how making extra principal payments on a mortgage enables homeowners to pay their loans off early and save thousands in interest charges. Then I got a note from a reader who worried that this would reduce her federal tax deduction for mortgage interest.

Yes, it would. But I wouldn’t let that be too big an issue.

Imagine that you’re in the 25 percent tax bracket and paid $10,000 a year in mortgage interest. Deducting the interest would cut your federal income tax by $2,500.

Paying the mortgage off would eliminate the deduction, wiping out the $2,500 tax saving. But it would save you $10,000 in interest charges, so you’d be $7,500 ahead.

Don’t let the tail wag the dog. It doesn’t make sense to spend a dollar just to cut your tax bill by 25 cents.

Nonetheless, the tax deduction is a factor in deciding whether paying down the mortgage makes sense.

As I said previously, paying extra principal is like earning an investment return equal to the interest rate on the mortgage. If the mortgage rate is 6 percent, eliminating that charge by paying extra principal is like earning 6 percent a year.

It obviously wouldn’t make sense to put extra money into a 6 percent mortgage if you could instead invest it in a mutual fund offering 8 percent.

But be sure to look at the alternatives on an apples-to-apples basis. That means an after-tax basis.