I have had my frame about 16 years, when i repacked crank bearings I also cleaned and repacked wheel bearings. You have to do in an area where you can track down a lost ball, but they run nice with some lithium grease.
I actually need to do that to my road bike wheel (though I suspect it won't be covered with mud). 13,000+ miles on the wheel, but during the last ride it made a weird grinding noise and actually slowed down the bike a few times. I'm hoping to open it up tomorrow or Saturday.