Well, FWIW, anything darker than 30% in Washington state is illegal on the front windows. The back...you can do whatever you want. SUV's and minivans come with 5% from the factory. Reflective tint is also illegal on the front windows in this state.
I had reflective tint on my front windows on my last car, had it for FOUR years, got pulled over twice for speeding, and the trooper never mentioned it.
Was NEVER an issue seeing out. Why would car manufacturers put such dark tint on the rear windows if it was?
I'm thinking of putting 20% all the way around on my new Subaru wagon, and my wife will probably do the same on her new IS.