If there is no contract, there's a good chance that the customer will assume, incorrectly, that they own copyright of commissioned work. If you are in any doubt at all, do absolutely nothing until you've been paid.
Then you can do what you like with them, legally, but the client may have a different view, rightly or wrongly, so it's a) polite, b) professional, and c) prudent - if you value future commissions - to keep them fully informed of your plans.
This should all be agreed up front. Regardless of the law, it can be a very touchy subject for photographers, clients and subjects.