I used to trim off my batting and backing fabric before adding binding. Not any more. It became too frustrating trying to keep all 3 of those layers flat and even while adding binding. Now I attach binding first, then trim off the batting and backing.
I know everyone has their preferred width for binding. My preferred width is 2.5". I use about a 3/8" seam allowance when adding my binding. When I trim off my batting and backing fabric I generally leave at least 1/4" to 3/8" of batting and backing beyond the edge of my quilt. Since I like a full binding, I find it's easier to trim off a little bit more as I turn my binding, than to wish it was fuller.