I clean mine when I'm expecting company, mostly. Sometimes even less often than that. Doesn't take much. Usually just some dusting, sometimes a little Windex on the glass, Lysol wipes on the controls, and make sure no cobwebs have built up inside.
I tend to play test it about as often I clean it, to make sure all the emulators are running as they did the time before. There's nothing more embarrassing than showing the cabinet off to a guest (or guests), and encountering some fatal video driver issue that you swear didn't exist last time you checked.