I've just had my TC 40D w/ SS for a week now, but I've been doing mostly loader work, trying to get a little better at using it. I've been mostly getting full loads and making tight circles to dump it back to the other side of the pile (I know its not very productive and my neighbors probably think I'm crazy, but it's been too wet to do much else). Anyway, I sure haven't seen my tires act like they are going to roll off the rims. They seem to stay perfectly upright and in good contact with the ground. I'd agree on checking the tire pressure.
The only problem I've had with SuperSteer is that I also got the quick detach bucket. Lining it up to reattach is a little tricky, as the front end moves in the opposite direction you're turning the wheel slightly. I had heard about this before and even test drove one for that very reason and didn't think it was bad, but I didn't try lining up with a bucket like that. I'm already getting better at it so I am still very glad I got SuperSteer.
--Brad