Anyone now receiving pension or annuity payments may find the checks smaller after Jan. 1. That is the day a new law on tax-withholding goes into effect.

Starting next year, federal income taxes may be withheld from your taxable pensions, annuities or other deferred compensation, just as taxes are now withheld on wages and salaries.

This change does not increase total tax liability. You owe the same amount in taxes whether you pay monthly (through tax withholding), quarterly (through estimated taxes) or all at once on April 15. But if you accept tax-withholding as a useful way to pay as you go, each pension or annuity check will be reduced.

You are not required to accept this new form of tax-withholding. The law allows you to demand each pension or annuity check in full. If you opt out of withholding, however, you may have to make quarterly estimated tax payments in order to avoid penalties. Penalties may apply if tax-withholding, plus estimated payments, amount to less than you paid in taxes last year.

In most cases, you will have to decide what to do about tax-withholding sometime next month. Your former employer, or the company that pays the pension or annuity, should send you a special form, asking whether you do, or do not, want taxes withheld. You must fill in that form and send it back.

If you do want tax-withholding, you will be able to specify the number of exemptions you expect, just as people do on regular wage-withholding.

Anyone who has elderly parents who might not understand the withholding form should provide help. If a new form is not returned, taxes may be withheld automatically as if the recipient is married and has three exemptions.

There are two exceptions to this rule: (1) No taxes will be withheld if the taxable portion of your pension or annuity amounts to less than $5,400. (2) If you chose to have taxes withheld under the old law, your company may, if it wishes, honor your old tax-withholding form even if you don't send in the new one.

Some people may have received the new tax-withholding form already and, not understanding what it was, thrown it out. If you think this may have happened to you, or to your elderly parent, write to the company and ask for another form.

Some companies may not be able to communicate with their pensioners or annuitants by Jan. 1, because of the time it takes to set up a tax-withholding system. In fact, the banking and insurance industries have asked the Internal Revenue Service to delay the start of tax-withholding until July 1, 1983. If the startup date is not delayed, and your company doesn't yet have its paperwork in order, you may be withheld automatically for the first few months next year, like it or not.

Don Tegeler of Hewitt Associates told my associate, Virginia Wilson, that most people are opting for tax-withholding. "They might otherwise be liable for filing quarterly estimated tax returns, and are afraid of penalties if they don't," he said. But a significant number of pensioners don't have to file estimated returns because they owe less than $200 in taxes. Don't accept tax-withholding unless you are sure that you will owe a tax.

Regular tax-withholding, based on the number of exemptions you claim, will apply only to periodic pension and annuity payments, such as those you receive every month. Nonperiodic payments, such as occasional withdrawals from an Individual Retirement Account, will be withheld at a flat rate of 10 percent unless you file the form rejecting tax-withholding.

If you take a total distribution from your pension plan in a single year, taxes will be withheld according to a special table not yet published by the IRS. However, if you ask your company to roll that distribution into an IRA or other qualified pension plan, no taxes will be withheld.

You may change your mind about tax-withholding any time you want. Just notify your company that you want to stop withholding (or start it, if you previously opted out). Your company will automatically remind you once a year that withholding on pensions and annuities can be changed, and will send you the form that is needed to change it.