I thought the reason for sanding diff rings was to not only insure they are flat, but to promote the diff balls rolling on the rings as opposed to them sliding across the rings which we all know will cause flat spots on the balls.

I have come to believe a stock diff can be just as smooth and last just as long as any after market kit. It all depends on how well it was built, and taking the time breaking it in properly. I learned that lesson after rebuilding my 22 diff about 22 times.