The highest ranking people in Washington have available to them, free, any time they wish, an expensive treat - one which, if they took it, would cover them only with status, not suspicion.

But they hardly ever bother.

The three presidential boxes at the Kennedy Center - 12 choice seats in the Concert Hall, eight in the Opera House and eight in the Eisenhover Theater, all with private bathrooms and refrigerators stocked with champagne - are there for the asking for the President, the Vice President, Supreme Court Justices, members of the Cabinet, senators, members of the House and the senior White House staff.

President Carter has attended eight performances at the Kennedy Center. By presidential standards, this qualifies him as being culture-crazed. But his party, including Secret Service, usually doesn't occupy more than four seats in one box, and there are all those other chairs available.

The Kennedy Center keeps them free for White House use, and early in the administration Mrs. Carter noticed that they often were standing empty, even at sell-out performances. So last spring, she established a sign-up sheet, and made it known that everybody on the White House staff was elibible to attend.

This includes the mansion's domestic staff. The next time you crick your neck to peer at the celebrities in Box One, you may be watching one of the two White House Mess stewards who take advantage of the privilege.

Mostly the boxes are used by "junior White House staff - press aides, assistants to special assistants," said Ann Anderson of Mrs. Carter's staff. There is bumping system, she said, but it is hardly ever exercised because of the lack of high-level demand.

Even when the Carters decided at the last moment to attend the Sarah Caldwell "Barber of Seville" staring Beverly Sills, an all-star major event in the opera world, they didnt' need to bump anyone. Only a few of the seats in the presidential box had been requested, and the Carters simply shared the box with their employees.

No Supreme Court Justice has requested it in this administration, few cabinet members have, and fewer members of Congress, said Anderson.

But the Kennedy Center has been criticized recently for being a place built by "the power elite" so that they - in the practiclly identical words of recent articles in Harper's and the New Republic - can "see and be seen."

"I had been going to concerts at the Kennedy Center for years, and I never knew these boxes existed," said Vivian Lichtman, now an assistant to a special assistant to the President, who often signs up to use the box for National Symphony Orchestra concerts. Nor does she feel "seen" there - she's never noticed anyone looking.

Ellen Goldstein, a balletomane who works for the Domestic Council, said she now looks into the presidential box if she's sitting elsewhere herself, "because there's likely to be someone thereI know."