For what it's worth, I worked at a local performance shop that tinted vehicles for a long time. In fact, one of my closest friends is the owner of it so let's just say I get pretty cheap labor when it comes to installing all my mods
Anyways....I have found that over the years doing one shade all the way around generally seems to look the best or classiest if you will. For example, the above poster did 20% all the way around (all the windows) I myself went 30% all the way around. Staying with one uniform shade generally looks the best no matter how light or dark you end up going.
Like others posted....different states have different laws as far as what you can tint and how dark it can be. However, as long as you don't go overboard most law enforcement officers have better things to do than pull you over for dark windows. It's a double edged sword though as it gives them a reason to pull you over whenever they want which gets people in trouble if you have had one too many beers etc.
Hope that helps!