Brad,
According to my dealer, who has been around long enough to know, they don't have problems with wheels rusting out. I've seen some very old wheels, which have little if any signs of rust on the inside..
what they do is lay the tire on it's side to fill it, that way, when you stand it up, the liquid is fully covering the wheel on the inside, so no air can get to the steel. supposedly, this keeps the rusting to a minimum..