As both a cop (NJ), and an M3 owner (tinted of course) I'll throw my hat into the ring here.
In NJ you can not tint your front windows. There is actually some confusion about this, even within law enforcement, because some believe that you can not tint any window in the aftermarket, and some feel that the front two windows are off limits, but the rest is fine (the NJ statute seems to address only the front windows, but I have seen documentation that bans all windows as well).
As for being ticketed or stopped for window tint, it is absolutely, 100% legal to stop a vehicle for just about any infraction, be it a moving violation (speeding, no signal, etc) or an equipment violation (no front plate, window tint, air freshener from mirror, aftermarket exhaust systems, etc.).
Having said all that, I haven't been stopped in my M3 for tint (40% - just liked the way it looked on the Space Gray) or no front plate yet. My sister's 335i has 5% tint, and I have gotten stopped driving that at least 5 times. I do work with several guys that will use window tint as a reason to stop a car and see if anything more illegal is going on, but generally if the driver is respectful they wouldn't actually be ticketed for it, or I might stop a guy for driving Mach 1 on the road, and rather than a 6 point ticket, just write him a $50 window tint ticket.
If you get stopped just stick to "yes sir, no sir, sorry sir" and you should be cool. Plus, most cops are car guys, so as long as you do something tasteful to your car, hopefully they will leave you alone