Yeah, but I'm thinking if it's in the gallery it shouldn't really matter. Even if the discussion is old, someone can still comment on the original piece being showcased. Bumping would really only be a problem if people continually bumped multiple old topics causing the newer topics to get bumped to the bottom. But if it's just once in awhile I don't see a problem.
So I'd say just try not bump old topics if you can avoid it, but once in awhile is fine.