I'm thinking on this very same topic. I had an 05 Dodge Ram with 5% all the way around, love love love loved it but was very dangerous. I never once got pulled over, here in Michigan you can only have the top 4 inches of the front windows tinted. It was so damn dark to pull out in traffic I rolled down the windows and and winter that sucked. lol
I agree with everyone that 50% is worthless and make sure you go to a good tint place because a bad tint job makes your car look like crap. If I do tint it will be 35%, that imo is a good balance between having tint and being safe. Please don't put different tint on the font and back, looks really bad but again just my opinion.
BTW, I was able to get my eye doctor to right me a note for tint, I know these things hardly ever hold up in court but it would be useful to get a cop off your back or slide you into a warning ticket. The Dr wrote me as having Photophobia.