BY ALL MEANS, let Congress add a nickel to the federal tax on a gallon of gasoline. Congress would have to take the tax up to 15 cents a gallon merely to restore the original value of the present four-cent tax at the time it was imposed in 1959. An increase in the gasoline tax is long overdue.

But Congress has a responsibility to pay attention to the cumulative effects of these piecemeal changes in the tax system. The structure of federal taxation is changing rapidly, among the various cuts and increases of the past two years. The taxes being cut are those in which people with higher incomes pay higher shares. The taxes being raised are those that are flat, or that impose higher shares on people with lower incomes. The gasoline tax is well justified, both as an incentive to conserve and as funding for highway repairs. But it is part of a larger pattern that is less easy to defend.

The great cut in personal taxes in this administration has been in the income tax rates. For people in the lower and middle brackets, inflation has offset some of the rate cuts. But for the people in the top bracket, the effect has been unequivocal. For them, the top rate came down from 70 percent to 50 percent, and that is not affected in any way by inflation. Congress made another and separate reduction last year in the inheritance tax -- again, not a change that affects the poor.

In addition to the gasoline tax, Rep. Dan Rostenkowski, the chairman of the Ways and Means Committee, was speaking last week of a further broad tax on energy -- another good idea, taken on its own terms. The Social Security rescue is likely to require an increase in the payroll tax -- and that tax, as every working person knows, is a flat rate up to a maximum of $32,400, with no exemptions and no deductions.

There's currently talk in Congress of a flat tax, as though it were a great and exciting innovation. But a decade ago, if you had added all the taxes together, including state and local property and sales taxes, you would have discovered that the flat tax was already the reality for most people. Except for the quite rich and the poor, there wasn't much difference in total incidence from one income level to another.

Since then, the flat structure has begun to tilt to the disadvantage of people on the lower rungs of the income ladder. Each of the tax increases now under consideration has its own rationale, and sometimes a very good one. But the traditional American idea of fairness holds that people with higher incomes ought to pay higher tax rates. The tax system, as it is now evolving, increasingly violates that rule.