Congress is virtually certain to include special tax incentives for savings in the tax bill now being written.

Boosting national saving now is seen by most legislators and the Reagan administration as an important aim of the tax bill. And while the administration started off with no special tax breaks for savers in its bill, they were included in the legislation introduced this week. More could be added by Congress before a bill reaches the president.

Despite the accord between Congress and the administration over the value of special tax incentives for saving, some economists, such as Joseph Pechman and Barry Bosworth of Brookings Institution, question whether they will indeed boost total savings and whether they will encourage the best kind of saving.

Most of the tax measures which have been proposed would give a special bonus to particular kinds of saving or would lower effective tax rates on interest and other income from saving.

None would require individuals to increase their net saving -- saving less borrowing, or realization of assets -- to benefit from the tax break. Without such a rule, some critics argue, it would be worthwhile for some people to borrow money from one source and invest it in a tax deductible savings account.

The present tax code is skewed in favor of borrowers through the tax deductions allowable on interest payments for both consumer and mortgage loans. These help make it profitable to borrow to reinvest in tax deductible forms of saving. They also, of course, tend to encourage spending and borrowing rather than saving. But the political constituencies for these interest deductions are such that even this savings conscious Congress and administration show no signs of tackling it.

The administration has proposed a substantial easing in the rules governing individual retirement accounts (IRAs) and retirement plans for the self-employed. The first would allow the 42 million workers who now have a company pension plan or retirement fund to open IRAs and deduct up to $1,500 a year in contributions from their federal income taxes.

It also would make it pay for many people to switch present savings into an IRA account to claim the tax deduction. Others might borrow money, from taxes and then reinvest the loan in an IRA and claim a deduction on the whole value of the loan.

In neither of the above cases would total private savings increase, however.

This is also true of proposals to initiate tax-free savings accounts, for example, for building up the down payment for a house or paying college tuition.

Other tax measures which have been proposed with the aim of raising savings include an exemption from gross income of some of the interest and profits on saving and investment. Reagan this week proposed making permanent the present dividend and interest exclusion of $200 for an individual and $400 for a married couple. It would otherwise have expired at the end of next year. Some congressmen have advocated a substantially larger exclusion.

Pechman commented recently that the exemption, which started on Jan. 1 of this year, has so far had no noticeable effect. Indeed the saving rate -- or portion of personal income which is saved -- has declined slightly since them.

As Treasury Secretary Donald T. Regan argued to congressional committees earlier this year, such exemptions are very costly in terms of additional savings generated because they give big tax breaks on savings that would have been made anyway. Present savers would receive a big windfall benefit which would not be paying for new saving.

It was for this reason that the administration urged Congress to concentrate on cutting marginal tax rates, to raise the after-tax return on savings, as on other income, from what it would otherwise be, rather than to enact various special measures.

However it now has opened the way for the special tax incentive measures, beloved of many congressmen and senators.