There was a discussion on here somewhere that it really depends on if your tush tends to push things out or pull things in. It seems the majority of folks have a tush that wants to push things out if left to its own devices. BUT, if you have one that wants to pull things in, you could run into trouble.
I suppose if you know your butt wants to push things out, and you keep a firm grip on it, you could use it. If it was me, I'd always worry about the slim chance of it going too far north and I wouldn't be able to relax and enjoy. I would suggest the add on bases that were mentioned above just to be on the safe side.