You do not want to leave a selvege on because it is a tighter weave and will make the seam pucker. Also, for the backing, you should have a half inch seam for each section. This allows for more give and wear and tear with the finished quilt. You could measure the selvege width add 1/2" and sew that amout as your seam width. After the seam is sewn, you can go back and trim off the selvege width.