My experience is only with Avis, but they share a lot of the same policies as they are under the same parent company:
I've complained a few times of the car smelling like smoke or gotten into the smoking debate with employees b/c of nasty cars. I've always been told that Avis only charges a renter if there is physical, visual evidence of smoking in the car upon return. So unless they find cigarette butts or ashes, they can't charge you. Seems absurd to me but that's what they claimed.