I have a 2007 Impala LT with 20% on all the windows. Looks mint. I don't like the way cars look when they have the back ones darker then the front two. Looks more streamlined when it's all the same. Been driving this vehicle and my last one which had tint all the way around at 20% as well for 4 years and no issue with the police, even after being pulled over numerous times for other issues.

 

35% always looks nice. any deeper and ur prob asking for trouble from the cops. here in NJ, if the front driver and passenger windows are tinted, they will bust chops. however, with a pba card, they will let u go. if u happen to have the badge in the window, they usually won't even pull u over. i have neither so i have kept my car tintless. however, my buddy had a green 97 maxima with 35% tints and his car look SWEEEEEEET!!!