I've always used an open stance for chipping, but I realized the other day that I had no real idea what distance I was supposed to stop opening my stance. I recently asked a local pro and he said he just does whatever he feels is right for the shot. That is essentially always what I have done, all the way out to even ~70 yards on occasion.
edit: Of course, I do use an open stance if I want to hit a cut/fade at any distance, but I'm just talking about the "short" game here.
What do you guys do? Do you have any set-in-stone rule that you follow?