Some people cut their quilt edge slightly wider to make sure the quilt "fills" the binding; they don't like a flat look to the binding. Some quilt show judges will mark off for bindings that are not "filled".
I personally don't cut the quilt to size until after I have sewed the binding on. I use permanent marker to mark the cutting line and place my folded binding fabric edge on that mark. It seems to me I have more control of the edge this way.
Here is a Youtube video for an "invisible" way of attaching a binding completely by machine. This is the method I am going to use on my next quilt, as I think it will be faster and still give an acceptable look. (I have tried other binding-by-machine methods that came out totally unsatisfactory for me.)
http://www.youtube.com/watch?v=wprg5vzkuGw