I have washed a rented car, but then again I was 25, my dad gave me the skymiles, I was off in the foreign land of Kaua'i Hawaii and I didn't want to have to pay the cleaning fee, so i deemed the $4 potentially worth it.
Then again I am a bit older, arguably a bit wiser, and I haven't even considered cleaning a rental car since then.
I admittedly don't know anything about squatters -- but if you were going to squat in order to own a house, why would you treat it like a rental? You're trying to take ownership, you would treat it like it was yours.
There must be some aspect to this that you all know about but I'm missing.
My brother owns rent houses; from the stories he tells, he would disagree even about renters, let alone squatters.
And recall the quote attributed to Lawrence Summers: In the history of the world, no one has ever washed a rented car. [1]
[1] http://quoteinvestigator.com/2010/06/21/wash-rental/