Depends on how happy the client is I guess, and the likelihood that they will be a return customer, if that is your goal for the project.
If it's neither here nor there, i wouldn't bother. Too much hassle, and there's no reuse. the only exception would be if its obviously bad sql that was hacked together to "make it just work" that are used frequently, or heavy javascript animations, i would move on and start a new project asap after reviewing the success of the one you are about to complete.
If you modify the code - you will need to retest it.
why not spend that time testing in the first place?
You might get more value out of refactoring the code, for your own use (if possible).
If your happy that all the requirements have been met - hand the project over to the client earlier than they expect.