It is so subjective. On a trip to Hawaii earlier this year, I had one final beer that we had not consumed that I left in the back of the car -- figured that the rental car cleaners might appreciate it and better than throwing it away. The guy who checked the car back in at Hertz noted the beer and said, "oh, you better take that out -- if you leave anything in the car, I have to charge you a $40 cleaning fee". :

The only time I ever washed a rental car before returning it was after some exploration on muddy roads in Red Rock Canyon -- the Yukon XL was mud spattered so to be safe I took it through a car wash. There were 3 or 4 other rentals being returned to LAS at the same time with much more mud and I didn't see the return agents blink an eye.

I did once return a car with some dog hair in it, and was given the opportunity to "vacuum it more thoroughly" before checking it in, which I did (there was a vacuum at the HLE) to avoid a charge. It was fair.