It's worth checking the wheels every so often by jacking each one up and giving it a waggle. There should be no play at all. Tightening up the end float often solves any problems, as AndyB says. If you overtighten, the bearing will wear prematurely, but you should be able to get a feel for what's right. (There's a scientific way to do it too, as AndyB also says, but I cba with all that...)