The big problem with using servo's as wheels for robots is the deadzone. Once you glue the pot down you're stuck with the 'center' position, and with temperature and voltage that center position can drift enough to cause the wheels to verrryy slowly turn requiring re-trimming periodically. Good for a quick project, but a more serious robot would be better off with one of those nice double gearbox/motor sets on the Pololu site, they're very reasonably priced. Only thing you need at that point is an h-bridge.