If sitting out is OK, when does it become OK? Right before the big rivalry game, if you're not going to make the CFP? Before the CCG if you're not going to make the CFP even with a win? Or if you're big underdogs?
IMO, if you commit to play your senior season, you play your senior season, to include post-season games. Your teammates are playing, your coaches are coaching, so why are you taking a pass?