Unless you have a complete history of what the previous owner did and when, I'm in the school of thought that "it may not be worn/broken now, but at least I'll have peace of mind if I replace it now".
Granted a wheel bearing is unlikely to fail, and I can easlily be shown to be worn (wheel wobble) grinding noise etc..
But it's one of those jobs that is a bit of a potch - particularly with rusted/siezed nuts.
I would say leave it until you are doing the brake calipers, but If you wan't to get as much stuff out of the way in one go, then do all four at the same time,as you may as well do the front as well.
Jonathan.