I have tinted many of my Cars and Trucks. I never had a prob with the law and I live in NY. I match the tint in the front to the back that way it all matches and flows. I think its a waste to not match the rear windows.
In IL, you can't tint the windshield other than the top 6", and tint can't exceed 35% on front windows or rear window. Some municipalities have their own rules, though. You can get a ticket just for driving through the wrong town. Fines range to $250.