I won't go back to a store that rips the fabric. Ripping cross-grain damages the fabric fibers up to 2 inches in from the rip. The damage isn't necessarily visible to the naked eye, but shows up under a microscope. Also, I find that ripping often stretches and distorts the ripped edge.
Now, at home, I will sometimes rip a fabric along the lengthwise grainline for borders. That is not as hard on the fabric, I think, as ripping crossgrain.