I like bindings to fit the quilt. Sometimes a quilt looks better with a larger binding, sometimes a smaller one.
So, I take a scrap, folding it like I would a real binding, pin it in place, and see how it looks. I adjust the width until I like it, and the seam and edge allowances fit properly. When the final binding is as wide or narrow as I desire for that particular quilt, then I measure and cut that.
Sometimes it does depend upon the fabric I have left, of course, but usually it's all about how I like the looks of it. Also, since I use many different battings, that plays a roll in the width as well.
Debbie in Austin