1) Make sure that it is something you want to do. Driving for the first couple of weeks with tinted windows in the dark is kind of a new feeling. Granted that you will get so used to it that you will not even notice anymore eventually, it's still something to think about.
2) Personally, I like tinted windows on lightly colored cars. The tints look darker from the outside than they and it looks good. Considering photoshopping a picture of your car to see if you personally like it. After all, it's YOUR car.
3) I have 15% tint and I really like it. It is not too dark to be uncomfortable or get you pulled over (at least where I live) but still looks dark enough from the outside.