I do not see it as a fine line at all... it is simple. Work with your customer to determine their exact needs. If they want perfection, and you promise perfection, you better produce perfection... and the customer should be paying for the time required to produce perfection.
It is wise to discuss the pros and cons of more serious defect removal (pro being removal of defects, major con being removal of more paint) and educate the customer so that they can choose what is best for them.
Personally, I never promise perfection as there is almost always going to be some defects that are too deep to safely remove. If the customer insists on pushing to remove every last scratch, they will most certainly be signing a waiver releasing me of any liability of any potential damages.... it has never come to this though because I work on mostly daily driven vehicles that simply do not need perfection. A 2-3 step process will provide an incredible enhancement on even the worst vehicles.
Moral of the story - set clear, realistic expectations with your customer before making the appointment.