If they start with a torn edge, measure and then tear your yardage, it seems to me you shouldn't loose anything. If it is "wonky", pull two opposite corners (diagonally across the fabric) and tug it until the threads shift and it squares up. My home ec teacher in high school taught us this. We had a real tug-of-war with some fabrics to get them to be on-grain; short, snapping tugs do better than one pull. Then you have all the treads lined up so that when you fold in half the salvages match and the ends are also even. I think it's worth the extra time for some fabric, especially plaids that are woven into the fabric and not just printed on top.