Every dog has his day, but should Top Dog's have been April 28? Presumably that question was on the minds of executives at LIVE Entertainment, the film's distributor, when the Chuck Norris flick opened in more than 1,900 theaters nine days after the Oklahoma City bombing. Top Dog pits Norris and his K-9 cop partner against militant right-wingers who plan to celebrate Hitler's birthday with a bombing. To dispel the impression of callous aforethought, LIVE issued a statement: ''We have decided to distribute Top Dog as planned because it is an action-comedy for the whole family.'' But Oklahoma City wasn't buying: On April 29, at General Cinemas Penn Square, one of five local theaters showing Dog, the film took in $300 (its national per-screen average was $1,133). ''The grosses have been pretty low,'' says Chris Mock, the multiplex's chief of staff. ''Priest, which is pretty downbeat and pretty controversial, is doing better.''