The main reason to trim off the selvage edges is that they are more tightly woven and will shrink in a different way when washed as well as being somewhat harder to quilt. I usually just take a nip in the fabric next to the selvage, then rip. If you feel you have to use that little bit to make the back large enough,then definitely take snips in the selvage every couple of inches or so, similar to easing in a sleeve in a garment.