Yikes. I do not purchase fabric at shops that rip. Ripping damages fabric fibers for a good 2 inches from the tear. Much of this damage is invisible to the eye, but can be seen under a microscope.
Straight-of-grain is very important for garment sewing because it affects the drape of the garment. It is much less important for quilting.
The biggest problem in quilting is distortion from bias edges. For people who do not prewash fabric, there is enough sizing in the fabric to prevent slightly off-grain cuts from affecting piecing. For those who prewash, all they have to do is add starch to the fabric before cutting. Starch stabilizes fabric.
I do not prewash fabric (I do check for colorfastness of suspicious fabrics, and will wash and starch a fabric if necessary to ensure colorfastness) and I do not worry about being completely on-grain with cuts. I heavily starch backing fabrics before layering too. Once moderate quilting is done to a quilt sandwich, the batting takes control of fabric shrinkage so again it matters little if pieces are slightly off-grain.
Many quilters come from a garment sewing background, and I think this is where the unneceesary concern about on-grain comes from. For quilting, it is enough to be reasonably close to on-grain. The only time I might start taking grain into consideration is if I were making a show quilt to win a prize when everything needs to be absolutely perfect.