Originally Posted by
Continuity
Indeed - I was just about to quote this old post of yours, Hillrider:
Originally Posted by
HillRider 
The "Grade" numbers for bearing balls are the deviation from roundness in .000001" (one millionth of an inch) increments. Grade 25 balls can be "out of round" by 25 millionths and Grade 300 by 300 millionths. Therefore the lower number is better.
Exactly right. Also willing to bet one would be hard pressed to find any of the cups or races on a typical bike that were machined to better than a few thousanths, so the balls aren't even close to the limiting factor... and given the low rpm, it is not worth worrying about much /K