Like most, I do not wash/vacuum the car before I return it, but I do try to tidy it up as much as possible. We just rented a car for a 10-day National Parks vacation. Oh my goodness, the bugs! We did not wash it (beyond washing the windows when we filled up with gas), but I apologized to the intake fellow that it was so filthy (on the outside). He told me not to worry about it at all. On the inside, we tried to keep it clean throughout the week (trying to knock the sand off of our shoes before we got in, etc.) and definitely threw all trash away before we returned it, etc. I can't believe people would leave trash in the car!
Similar with hotel rooms, I try to leave them reasonably tidy. If we have trash that won't fit in the tiny trash bins, I stack it somewhere so it will be as easy to clean up as possible. I think you're paying the hotel staff to clean the bathrooms, change the towels, make the beds, etc... but not pick up your trash from all over the place.