When I worked part-time for a nationally held fabric outlet, we had a good part of our business in quilt fabrics. We did not tear, but tried to cut accurately and add a little for assurance. Sometimes an obvious stripe or print was not printed on the straight grain, so a choice has to be made sometimes--do you want straight printing or straight thread cut? For small cutout pieces, grain may not matter a good deal, or one can make adjustments easily, but for larger cuts (like the back of a table runner), one would want good accurate thread eveningness. Rotary cutting can be more accurate for on-grain, but don't expect much more than what the sales person can and does with scissors. Frankly, the reason we were given for not tearing (a former common practice with cottons) is the loss along the torn side that customers did not want (and I am sure that the company did not want to allow for by those 2 more inches expected) I was surprised at how often a customer was not aware of the practice of pulling a guiding thead to cut through for absolute grain accuracy When wanted.