You can tint the rear sides as much as you want. I have limo black on all rear windows.
The legal limit is 15% on the front (i think). Glass has a natural tint anyway, with some manufactures pushing the limit on this tint.
I had my windows done professionally. I clearly remember he stated, that a clear film on the front windows would push them to their limit. And he would not put any tint on them, as even the lowest grade of tint would make them illegal.
Any tint on your front windows will probably make it illegal, it's your own choice what you put on your car. But if you do the fronts, check with the company that is doing it, as some may not.