IN ANNOUNCING that he will impose between $50 million and $75 million in spending cutbacks starting July 1, Gov. Robb is at once following custom and modifying it: in even-numbered years, you talk shortage, while odd years are for surpluses. The difference this year is the swiftness of the warning and the targets of the cuts.

The justification for the cuts, as usual, is a projection of a shortfall in revenues--which is every governor's alternate-year citation of a gloomy report from the financial estimators. But Gov. Robb is candid on this point, noting that he has deliberately focused on the lowest of the revenue estimates, that "we'd prefer to be safe rather than sorry." And state officials point out that continued high unemployment and sagging income and sales tax receipts are reason to forecast "low growth" of 10.1 percent in revenues.

The other side of this coin, of course, is that anything better than the lowest estimate will look good and, if past is prologue, may look good enough to be called a "surplus" in due course. But already there is another noteworthy difference: Mr. Robb has excepted education and welfare from an otherwise across-the- board budget reduction. That makes sense, given the realities of federal cuts and limits to which assistance for the state's poorest people can be trimmed.

Just as unpredictable as the revenue estimates is the possibility that even with these cuts, taxes will need increasing. No governor relishes that, nor do the legislators. But the best insurance has to lie in preemptive prudence--which is what Gov. Robb has recognized and addressed early on.