I'd like to add that a good, serious cleaning also helps shifting and chain performance.
what I'm talking about here is putting on a pair of old pants and ratty t-shirt, getting out the rags, the old toothbrush and WD-40 and going to town on the scrud that builds up on cogs, deralleurs, chainrings and chains.
I personally have gotten jaw-dropping improvements in ride, and shifting smoothness by doing such a deep cleaning, plus, the bike is much easier to KEEP clean once this has been done.
with careful application of persistence and elbow grease, you can pretty much return your drivetrain to a nearly new condition, and you won't believe how well it performs afterward, even on a so-called cheaper bike.
*Note: your goal is not to liberate bearings and races of their grease..you want to remove grime from the exposed parts, then re-lube them.
one indicator I use to tell me whether I need to do some cleaning, and whether I have done a good job is how my chain and gears feel. If I don't hear or feel them during pedaling (not including shifting, of course) then they're properly cleaned and lubed. If I do, I know I have some more work to do.
but you also can't beat good old eyeballing to check drivetrain cleanliness, and should do so after every ride.
geardad