AS IF IT WERE a rite of municipal passage into some fiscal hall of fame, a mayor's budget is invariably announced by the media as having been "unveiled"--even when anybody who hasn't been treated to a peek already can guess correctly that there will less to it than meets the eye. Mayor Marion Barry's municipal budget for the fiscal year that will start Oct. 1 is no exception, even if its assumptions hold up. While the mayor's balancing of spending proposals against revenue hopes is anything but airtight, his budget is certainly no reckless call for a community bath in red ink. The key--and the proper focus of the D.C. Council in its deliberations--is the strength of Mr. Barry's revenue projections.

Mayor Barry is counting on inflation and a 12 percent increase in real estate values to raise $130 million above current levels, without--and here's the old elixir of election year--any increases in property or income tax rates. The critical word here, of course, is rates, since incomes and property values are expected to go up, which will raise the dollar amounts people pay anyway.

Is 12 percent a sound projection? Even the mayor's experts note that there are many uncertainties in any economic forecast these days (see For the Record, elsewhere on this page), given uncertainties in the national economy. Still, the 12 percent estimate does represent a moderation in the city's rising property values, compared with current assessments showing average increases of 18 percent.

As for spending proposals, they don't amount to any government shopping spree; the budget of $1.76 billion would be 11 percent above the current-year total. The largest proposed increases would be for jobs, housing, programs for the elderly and health and other care programs over which the city has little control. Cuts include a 54 percent decrease in spending for capital improvements, with the public works spending to be concentrated on streets, bridges and water and sewer mains.

Too many on the payroll? After layoffs of 1,000 workers in fiscal '81, Mayor Barry plans no more, but still proposes to cut out 768 more jobs through attrition and elimination of unfilled positions. The payroll? At a 5 percent increase, the budget would not be out of line.

But what if pay increases turn out to be higher? Or if property and income tax yields are much lower than projected? Or if other revenue increases based on a tax on utility companies' gross receipts and automobile and permit fees do not materialize or measure up to expectations? And what about the mayor's earlier emphasis on a start at eliminating the city's accumulated debt?

While Mayor Barry's budget may be optimistic, it is not unreasonable. Still, these are questions that members of the council should raise--because even though Mr. Barry may not have hard and fast answers, taxpayers deserve to know what the worst case could mean for city services and their share of the bill for them.