Can your dentist withhold your permanent crown due to nonpayment?

No, they cannot withhold your crown if they have already prepared the tooth. They are obligated to see the procedure through to completion once it has been started, regardless of payment status. However, this does not mean you do not have to pay or can get away without paying. If you do not pay, they will go through a collections process, or possibly even sue you for non-payment of services rendered. It is generally easier just to arrange payment terms than to deal with collections, legal concerns, credit concerns, and possibly wrecking whatever relationship you have with your dentist.