UML, or any other sort of high-level design planning, is a bit against the hacker spirit so my guess is you wont find many here who use it. Hacking is more bottom up, start with something simple and just keep iterating it into something larger.

The only instances I've seen UML being useful is in very large software projects with many people involved. Having an abstract way to convey program structure can be useful there. But for anything less it will probably slow you down and restrict your freedom, IMO.