I would always leave a bit extra even when I am doing the stitch right sides together and turn cuz if the batting or backing shifts even a little, you could end up ripping and redoing. It is much easier to trim off the excess than to rip, add more and redo. I often leave extra on the backing; I find I can easily add the excess trimmed off to my stash of scraps or even bigger cuts sometimes. The excess I trim from the sides of the quilts I often use for the binding of the quilt or sometimes even for a different quilt. I never goes to waste.