I think the legal limit depends on the state laws. Here in WI the darkest tint you can go is 45% (legally, mind you). 45% to me isn't dark at all. When I get my next car, I'm going to go 5% on the rear windows and 15% on the fronts. I think it's mainly so cops can tell what's going on inside the car (If you're pulling a gun on them, etc). Check with your state laws to know what the limit is for tint and then base your choice on whether you could be satisfied with the legal tint, or if you want darker. Going darker has it's consequenses. If you can pay the tickets, go with 5% (which is dark). If you're satisfied with leagal limits or don't want tickets, go with the darkest legal amount you can.
Personally, I prefer dark tint on any color car, but black and white always look best with dark tint IMO. Hope that helps you out a bit.
1999 Chevrolet Camaro SS (PC'd)
1994 Chevrolet Beretta Z26 Q4 (W41 M90'd)
2004 Chevrolet Silverado 2500 (4WD)
My wife's cars:
1971 Dodge Charger SE (stock and original 383 auto)
2004 Subaru Impreza WRX (kid-hauler)