I've always just left a "thank you" message to bump the thread, but I just stumbled upon one guy who was asking for rep. Are you supposed to bump the thread AND leave rep, just do one of those things or do something completely different?

You could just keep doing the same, you don't HAVE TO leave rep. Of course posting thanks bumps the thread, leaving rep doesn't. If you think they should have rep for something or other than by all means go ahead, but really you don't have to. Saying thanks is all right... in my opinion.

Oh yeah there is no rule that you have to leave rep.