My PS3 is COVERED in dust.....when I get around to playing it again I will dust it then. Or my wife will clean and notice the glossy black is a faded gray and take care of it.
360 gets a bit more attention though. All the other systems are away, in boxes or drawers.
When I get an older system I try to clean as much of it as possible.
If its cartridge based and i feel confident enough i will take it apart completely and go at each piece with windex, goo gone and Ammonia for the connectors.
My top loader NES looks almost new after i got the crayon, marker and what im going to assume was gum from the cracks and crevices.