On a couple of occasions I have managed to find the one available space left in a car park . When I have returned to the car I suddenly realise why I've been so 'lucky' , a quick look at the car and immediately the bird-roost above, confirms why no one else parked there.
In these circumstances I do clean the car before returning it.
However I've never been charged by Hertz (or National for that matter) for car cleaning , whether this is dependant on CDP and the amount of business that comes from my company, could be a factor. Some cars ,especially in the Winter, go back caked in grime.