New rifling, or old rifling, it doesn't make much difference at the ranges you will be shooting, or at least the difference won't be noticeable I would imagine, and the most important thing, just as you said, would be to find the slug that your gun likes, and shoots the best.
Once you sight in with a certain brand the poi shouldn't change enough at 75 yards and under that you would even notice that difference either.
Most people sight in every year anyway just before the season just to make sure the scope didn't get bumped and knocked off zero anyway. It's just cheap insurance to fire off a couple rounds rather than missing that once in a lifetime trophy, and wish that you had shot it before you went hunting.
I've had good luck with Remington Buckhammers, and the Hornady SST's are accurate at 100 yards out of my gun.
I like how the Buckhammers performed on the two deer I shot, but I haven't had a chance yet to test the Hornady's on game yet, but I'm hoping that will be soon.
I've also got the thumbhole stock on my 20 gauge slug gun, and it's one of my favorites.