Just my two cents.
I leave about 4 inches (first quilt was too short, so I learned this lesson via the hard way :?

I usually cut a 2 1/4" strip for binding and fold in half (iron flat) and then sew to the edge of my quilt. Once my binding is attached I lay out the quilt and trim a little over 1/4" b/c I like a full binding. You can always trim if it's too bulky (as you hand sew the binding down).

Hope this helps.