Whichever way you tear it whether it be width ways or length ways it will tear straight, I agree some polyesters will not tear easily but if we are talking bout cotton here, then I reiterate the weft runs one way and the warp runs the other it will tear easily and straight I have just done it to prove that I am not talking out of my head. both ways long and short.
Its the way many professional and trades persons will do it.
Elle