As important as the new faces behind the desks are to the grand scheme of things in Washington, the openings that languish unfilled nearly a month after the inauguration are as much so -- and possible just as indicative of the new administration's strategy.

Key sub-Cabinet spots are still vacant. Dozens of regulatory agencies or regulatory branches of departments are without leaders. And some of the government's biggest independent agencies have been told they are on the lower end of the priority list and can expect to muddle along with interim administrators and acting heads for some time.

Environmentalists are anxiously awaiting the announcement of a new chief at the Environmental Protection Agency, where there are enough rumored candidates to have a cocktail party without inviting a single lobbyist. rThe science community wonders when President Reagan will name his White House science adviser, not to mention when and with whom the administration will fill high-level jobs at the National Aeronautics and Space Administration, the U.S. Geological Survey, the Food and Drug Administration, the National Oceanic and Atmospheric Administration and the National Science Foundation, from which four key assistant directors recently resigned.

The reaction from the White House seems to be: All in good time.

There are, of course, some good reasons for leaving the positions temporarily vacant. First, there is the idea that budget-cutting can be carried out with less strain if the funds can be snipped before a turf-protector has taken up residence and has had his or her ear bent by the appropriate interested parties.

Then there is the need to find just the right candidate. In its Mandate for Leadership, a hefty tome that apparently carries some weight with the Reaganites, the Heritage Foundation urged the administration to act promptly to fill regulatory agency vacancies, but also cautioned that "because the independent agencies are creatures of Congress [and] the president has little leverage over their actions," it is of overriding importance to find people of "friendly persuasion." In fact, philosophical compatibility is the single most important qualification (the foundation's own italics).

Add that criterion to the administration's insistence that it is actively seeking women, blacks, Hispanics and other minority candidates, and it's little wonder that the cobwebs are growing in some important offices.