I generally cut my binding 2.5 and use a 3/8ths seam allowance when I sew it to the quilt. I generally use Hobb's 80/20 batting and it has just a wee bit of loft and fills the binding nicely at that width and seam allowance. For thinner, flatter batting I'll cut the binding 2.25 or even 2 if I want it narrower.
At my guild we had lectures two months in a row and the first teacher said she always cuts her binding 1.75 and the second one said she always cuts hers 3 inches.
Experiment. See what look you like for your project.