The advantage of leaving it open ended in your contract saying "responsible for any fees I incur as a result of an NSF cheque" means that if you have a payment bounce and are charged a penalty because you thought you had enough money in your account to cover the fee then the parents are responsible for paying your penalty since it is their NSF fault not yours.

Something to remember when making a contract - it is good to be specific so there are no misunderstandings but don't make the wording so rigid that it doesn't cover all possible scenerios.