If everyone tipped bellmen $2 per bag, they would make more than management level employees. 10 pesos a bag is generally sufficient. In most of Mexico, 10% at a restaurant is pretty standard in terms of what the locals tip (I live here). In resort areas that get lots of Americans, however, they expect 15% or will add that to your bill automatically. Locals tell me you should only tip a cab driver if they help you with their bags, but I follow the same policy on whether they've tried to rip me off or not, usually rounding it up if not.