Hi Beverly...
I actually add a good 4" to each side when I cut my backing and here's why...
1. When you quilt with a walking foot, the foot pushes out the quilt top a bit so it looks like the back is shrinking, when really the top is growing a bit...
2. If I'm going to be quilting for a long time I wrap the backing around to the front to protect the edges as I pull it in and out of my machine.
I've been trying to work down my stash a bit, so a lot of times now, I'll piece the back. Nothing fancy just a bunch of squares or rectangles, or I cut lengthwise grain strips (wide) from several fabrics (so I have a lot of use-able fabric of each left) and piece those together to make the backing. I use 1/2" seams and press them open.
I know what you mean, even though my stash is pretty embarrassing, I still hate to just waste the fabric.
Julie