Jim,
I may not be understanding the question. Are you saying which side you are supposed to sharpen? If you are, one side is sharper than the other cause one side got abused more than the other. If the other side is still sharp the rental store needs to flip them and use the other side. Because of the other side not being dull as often the blades probably are longer on that side. Therefore needing to adjust the anvil everytime they flip them. If i misinterpreted please rephrase your question or give more detail. BTW- what are you using to sharpen them?