TR -
NO, don't go forever. As has been discussed, there is a whole back side out there that you can keep us apprised of. And there are billions of full, round butts out there you haven't seen yet (I don't think).
Rimming is definitely one of those talents that you work on. You always want to please the guy you are with. I start out slow and try to take it deeper and make it more and more passionate.