Use a brass drift so as not to damage anything when removing/installing races. (which you don't need to do if you are merely servicing the bearings)

Inspect the bearings carefully after cleaning. Any visible pitting on the rollers or race is grounds for replacement, as is a series of radial ridges on the race. (if they have gotten to this point, you will usually notice a noise while driving)

Make sure to remove every last bit of old grease from the bearings before repacking. Greases can have several different formulations, some of which are incompatible.

Never spin-dry a bearing with compressed air.(it's OK to blow it dry if you hold it so it can't turn)

I've never seen a rear bearing go bad. The only reason I've ever had to replace them is when the seal has failed. (the bearing has to come off the axle shaft to do this, which usually destroys it)