Reminds me of the talk "Fuck you, pay me", where they talk about how the client only gets copyright assignment upon reciept of final payment. Thus, if they stop/don't pay, then they do not have copyright on the work, and hence are guilty of copyright infringement if they use it.
It is always a good idea to keep ownership until final payment. This also increases the chance that you'll see money when a client goes bankrupt and is sold or undergoes a restructuring ("doorstart").
