Lets not forget the importance of having balls of all the same size. It is better to reaplace all the balls at once than one-by-one should you lose a few cleaning the diff.

If you have 12 balls and rings that have a few months of runs of them. heaven forbid that two rolled off the table when reaplacing the worn spur gear. Don't just slap in two new balls. You'll be better off to just leave those holes unfilled or totally replace the balls and rings. The reason being is that with two new balls, those balls will be slightly larger than the remaining 10 balls and the contact pressure will be more on those balls than the others. Resulting in a hard to tune diff. A little too tight, diff is sticky from those two balls digging in, little too loose, diff slips.