For me it depends on how I am shooting. If I want precise hits and really tight groups I use my dominant eye, other eye closed. Yeah it can cause tunnel vision, but when you're trying to do your best at the range nothing is shooting back.
If I am combat shooting I don't necessarily try and get a real sight picture at all, at least that's what people accuse me of. I aim in a sort of Kentucky windage that used to drive my weapons instructors nuts. However, since I can do this and put all the rounds in the kill zone, and even get head shots clean, they let me get away with it.
If I am facing one opponent I can run a drill on him fast enough that he'll have two to the body and one to the head before he gets off his first shot, so I guess I must be doing something right.
I also do what is now-a-days called zippering. We called it stitching when I was younger. Which is, both eyes open looking at the target, put one shot you know is low in the torso and then walk the rest up to the head actually using the each new hole as a reference for the next shot and not sighting precisely down the barrel.
If I didn't explain any of this to someone else's satisfaction, oh well, it works for me. I hit the target in the red consistently which ever style I use.
Shoot as many different ways as you can as often as you can so that you are versatile if you find yourself in a drastic situation.