I don’t know of a standard, but things I consider are:
Is the drain line in the way on one side, and do I want to spend $$ modifying plumbing?
Will the drawers block a doorway when opened?
Also, drawer beside the toilet makes a good place to store a spare roll of TP. We had a customer with a cat that destroyed TP and ended up putting it (the TP, not the cat) in the drawer.