I read with great interest and some dismay Robert J. Myers' proposal {"Social Security Roller Coaster," op-ed, Aug. 20} to freeze the current Social Security tax rate. Myers would keep today's rate of 5.7 percent (Medicare takes an additional 1.45 percent) until 1996. He advances this proposal because he believes that the large fund increases produced by present law over the next 35 years under the middle-range actuarial projections are undesirable.

I disagree completely with his proposal to prevent the 1988 rate increase of 0.36 percentage points from going into effect. He has raised a legitimate question as to whether, in the long run, Social Security financing should depend in part on a large earnings reserve or should instead rely almost entirely on current taxes, backed by a contingency reserve to meet unanticipated changes in economic conditions. But one's stand on this question should not govern one's position on the 1988 contribution rate tax increase. Substantial increases in reserves are needed to get the trust funds back to a safe contingency level, even for a pay-as-you-go purpose.

Present reserves are inadequate by any standard. As Myers points out, they equal only about three months of benefit payments, whereas for safety they should be equal to about 12 months of payments. The first priority is to be sure that we get quickly to at least that fund level.

Although the middle-range estimates, which are probably the most likely, show a consider-fund buildup even without the 1988 increase, for safety we need to consider what would happen if the economy did not perform very well during the next five to 10 years. The more pessimistic projections of the actuaries show that without the scheduled rate increases, the reserves would continue to be dangerously low. In fact, under these assumptions, the ratio of the trust funds to the next years' outgo drops from about three months of benefit payments at the beginning of 1988 to about two months of payments in the mid-1990s.

If the more pessimistic assumptions turned out to be correct, without the 1988 increase the system would be vulnerable to a major economic downturn. Moreover, the pessimistic assumptions do not assume a recession of the size of 1981-82. If that occurred, the failure to implement the 1988 scheduled rate increase would quickly produce another Social Security financial crisis.

Before Congress and the American people were assured by the National Commission on Social Security Reform that its "rescue plan" would do the job, the Commerce Department, under Commission Chairman Alan Greenspan's direction, made one computer run after another simulating various possible economic conditions through the 1990s. The plan looked good during this period even under very pessimistic economic assumptions. With Myers' proposed changes, this would not be the case.

In its 50-year history, Social Security has had two financial crises, one in the mid-'70s and one in the early '80s. In both cases, the short-term crises were caused principally by inadequate reserves and reliance on economic assumptions that turned out to be too optimistic. We cannot afford to take a chance on a third mistake of this kind.

It will take us several years even under the contribution schedule in present law and with good economic performance before we reach a safe contingency fund level. During this time we can debate the longer run question of pay-as-you-go versus a large earnings reserve. The issue can be studied again by the statutory advisory council to be appointed in 1989. If we decide on pay-as-you-go and if the middle-range estimates turn out to be the correct ones, we can always cut back in the 1990s, as Myers advocates. But for now let's make sure we have a safety margin for the immediate future.

The 1988 contribution rate increase recommended by the president's bipartisan National Commission on Social Security Reform and as decided by Congress and the president in the 1983 amendments should be allowed to go into effect. The writer was Social Security commissioner from 1962 to 1973 and later was a member of the President's National Commission on Social Security Reform.