I've been fortunate to have a location close by. The sales people have been knowledgeable and I've asked them to help select some skins. They would explain why certain spots are ideal or not between the various grades at the time. If I were buying online unseen, that would be an unknown iffy variable.
I have some vegetable tanned stuff that was part of a batch lot that while looking good is inconsistent. There is one spot the person cut through.