It's important for the faces of the rings to be flat, I agree, but it's also important for the faces of the ring to be parallel to each other. I'm not of the opinion that hand-sanding can achieve that. On the other hand, if the rings are stamped from a reasonably flat piece of metal to begin with, sanding may indeed remove any high spots induced by the stamping/heat treat, but still not introcude any taper (unless someone sanded on the rings for a week straight...)
Now if someone were to put the rings on a surface grinder and make dead flat dead parallel faces, I'd be buying those rings exclusively. They'd be expensive, but if you're not slipping the diff a set of rings lasts a LONG time.