Yes, check with your quilter as to her preferences. If you really need to have that extra width you may consider this method, and again your quilter may or may not like a diagonal join in the backing.
I have never done a diagonal seam on a quilt but I guess I will try it.
If it doesn't work out, I will have some "new" fabric to cut sashing or squares from.
Thanks to you, Gay, for your help!
This must be the way to do what I need since you echoed what IceBlossom suggested.
I always get the best answers from this site!
Merry Christmas!