We are in Chicago and plan on doing 70% on the front windshield and either 35 or 30% on the front sides. I usually do 35 all around, but with getting white seats, that extra 5% might even out in the long run.
As for winter, I have tinted all of my cars the same and I honestly can not notice a difference. I actually feel heat stays in better during the winter months, however, yes you do not get the added sun heat. Then again, its winter and we get about 10 days of sun usually during those months. LOL