Ive had 18% all the way around (except the windshield of course) which is decently dark since 2007, got it put on a week after I bought the car, and never had a problem. Ive driven passed countless cop cars and sat beside them at traffic lights. My car is black too so that makes it look even darker. When its even a little bit overcast you cannot see in my car from the side at all (I guess I'm one of those idiots...but DANG does it look good!). I lived in NC before that and 18% was illegal down there too, but not "as illegal" (lol), and no problems. Got pulled over twice down there and cops didn't say anything.
It all boils down to the cop.. I dont think many will flash you strictly for tint but I guess some will. Ive already accepted that if I get a speeding ticket or something I'll likely have a tint fine to go along with it.