I live up in CT and all of my cars are tinted 35% sides and 20% back even though CT weather isn't scorching like down south. When I rent a car or get a loaner car from my dealership, the sun actually drives me nuts while driving. I generally only rent a car for a few days so I wouldn't get it tinted, but if I was renting a car for say 2 weeks straight or more, I would seriously consider getting the car tinted with basic tint.
That being said, I hate driving dirty cars and love driving pristine condition cars but can deal with "clean" cars. I have, however, done a full interior and exterior detail on a rental car before if I'm near home. To each their own, but for me personally, I would put another $100 to get a rental tinted or spend 2 or 3 hours detailing the car if I have the car for a decent amount of time.