Its only inside that needs it. (if be cleaning.)Most hire cars now, wont allow smoking or pets inside their cars.
As for it being dirty on the outside, why should you wash it when they have their own people to do it. It came to you Dirty, it can go back dirty (unless you been in dirt tracks etc ;0) )
You have paid enough to the hire company to hire said car, if they cant deliver a clean car to you (outside.) do not clean it for them anyway. you be depriving some-one out of their job.
When any time i have had to have hire or courstey cars if my car been off road, i never clean it on the outside nor inside for that matter as its kept clean on inside as with my own car.
Dont bother sheps, all they worried about is that inside of the car is clean and ready for the next customer.
That next customer may just get filthy anyway, so all your hard work be wasted.
They bump these cars out like a female guppy bumps her babies out. (Fast.)
If you really want to clean, if its still very cold, never use warm water, It will freeze almost straight away. Use cold water. (all depends what time cat goes back.) if its early, leave it be sheps. but if it midday etc and temps have gone up and sunny like it is here now, just throw couple buckets of cold water over worse spots, I.E. wheels. Me personally wouldnt bother.