There is a line on your windshield called the AS-1 line. This line basically tells you how far down your window you can tint. If a cop or state trooper see your tint past that line (any state, as far as I know) they have a right to pull you over.
EDIT:
I just looked for you:
http://www.tintcenter.com/laws/TN/
Windshield Non-reflective tint is allowed along the top of the windshield above the manufacturer's AS-1 line.
Front Side Windows Must allow more than 35% of light in.
Back Side Windows Must allow more than 35% of light in.
Rear Window Must allow more than 35% of light in.
Hope that helps