NOBODY SLEPT HERE: The White House circus has been hopscotching around the margins of the San Fernando Valley for several administrations. But when President Clinton sat still for questions in Van Nuys on Tuesday (A1), it marked the first time a president had spent quality time in the Valley proper in many years. . . . President Bush dedicated a jail in Castaic and stumped for Gov. Pete Wilson in Thousand Oaks. The most notable Valley-area presidential parade came when the Reagan Library was dedicated in Simi Valley in 1991, drawing Bush and former presidents Reagan, Carter, Ford and Nixon.