I've always used 12s. I echo the idea of tailoring loads to the birds. I used to use lowbase 8s for the first shot at pheasants over dogs. The bird was close, my reactions were fast, and the regular load of 6s just didn't spread enough to hit the bird.

What I have found most important is to make sure that the gun fits properly. I added a recoil pad to my newside by side and went from missing to hitting.