Even on the shortest DVC point stays, there's one full cleaning associated with your stay. You can either think of it as the cleaning that took place before you checked in (cleaning up to have the villa ready for you) or the cleaning that will take place when you check out (cleaning up after you).
We tend to leave some cash on the master bedroom pillow on check-out day.
Short answer: Tip housekeeping if you want to, not because you have to.