Graffitiing your boss' car and house because he sexually assaulted your girlfriend at a work party? Good. My mate did that, and I fully stand behind his actions.
Graffitiing a site you go visit? You're a dickhead who's just ruining it for everyone else. Same goes with smashing stuff up. It's unnecessary. However, depending on the size and nature of the place, a tiny little tag the size of your finger won't hurt too much. Unless you get caught, and all of a sudden you're in criminal territory.
I believe the general rule is take nothing but photographs, leave nothing but footprints - or something like that.
Graffiti on a random wall on an underpass? Most of it's shit but if someone has clearly got artistic talent or it's funny then fair play in my opinion.