Tooth contouring and reshaping is always safe and helpful, as long as it's confined to the enamel
= the outer layer of the tooth.
Thus, the only guideline is to "do no harm." this is easy to adhere to. Slight reshaping and polishing of teeth to make them more attractive is safe, effective, and easy. Like i said, you can only get into trouble if you remove all the enamel, which no one would do.