If you have developed a mutually trusting, respectful relationship with your daughter and have conveyed and lived out your family's values by your own example, then you have done the best you could do to prepare your girl and yourselves for her eighteenth year and all years to come.
If your daughter has lived with receiving appropriate natural consequences for her actions then I see no need to change your philosophy of discipline. If a written contract is something that makes you and your daughter more comfortable then by all means mutually draft one. I would recommend, however, that the most important aspect of writing a contract should be in talking about how all of you plan to love and trust and respect one another in the coming year; the contract shouldn't be a document that parents hold over their kid's head like a parole agreement.
Enjoy the joys, chaos and growth during this important age and stage of your daughter's. Stop work on the ark and send the animals home! Thanks for writing.