this was the subject of an extensive thread, which i cannot find now.
the upshot was that some us felt like the weaving threads were straight and tearing at the shop gave truer lines to follow. these members felt that shops ought to rip fabric instead of cutting.
others of us felt that cutting was better because fabrics today are not woven as evenly as they once were and after washing they will return to the shape in which they were woven.
it was generally agreed that ripping lengthwise was preferable for long strips as the lengthwise threads were reliable for borders and sashings.
if anyone can find the thread, that might help. but the results were really very mixed. i wish i could remember the name of the thread.