tearing your fabric makes a straight edge because the tear runs along the grainline of the fabric, usually after it is torn you then fold it in half salvage to salvage lining up the torn edge and trim it...you usually save alot of fabric doing it this way. i have bought cut fabric from the lqs that was as much as 3" off on one end; alot of waste squaring up my fabric, when the fabric is torn you only have to trim a sliver (like a 1/4") instead of inches. you have a straight edge to start with and less waste.