Should I replace all wheel bearings when only some are worn? I just ripped apart my mom's mountain bike's rear hub.  One of the bearings was incredibly rough, so I replaced it, and the other one was smooth, but didn't turn very easily.  Since I didn't have a replacement for it on hand, I just put it back, but I'm not sure if I should try to replace it.  Is there a good rule of thumb on bearings?  Does one failed bearing imply that another one is likely to go soon?  

While we're at it, should I be doing anything to the freewheel other than cleaning it and re-greasing it?  I've heard it's important not to put too much grease inside the freewheel, or it'll increase drag when you're coasting, so I was careful of that.  

The specifics for this bike: 


Mavic wheels
Shimano 9-speed freewheel
Full suspension mountain bike that gets fairly heavy use on XC singletrack terrain
According to Lennard Zinn's book 'Zinn and the Art of Road Bike Maintenance', you should replace all of the bearings even if only one is obviously bad. That way you ensure round, smooth, consistent bearings. I've been using his repair manuals for years and he's pretty much never steered me wrong.