I'm always leary of paid chemical maintenance. I consider myself to be pretty knowledgeable when it comes to adding chems and how much time to wait in between. If you're water is out of whack, technically speaking someone could be there for the better part of a day to properly add the chems for you. How can they do that and still make money?
They don't dump and refill on rentals? Am I naive to think that? ( I think I know tha answer)
I had assumed, when a new renter came in it was a clean fill, and put in clean filters. In the off season, either winterize it properly or do spot checks on it and keep the water clear.