There are 2 problems with tearing: #1. The fabric is not always woven straight. Or perhaps it gets stretched off grain when being rolled onto the bolt - something that happens by huge machines at the mill. You can't count on getting fabric square by tearing it.
#2. Tearing the fabric damages the threads at the tear. The thicker and better quality the fabric, the more this is true.
I am a professional long-arm quilter and I try to discourage my clients from ever tearing their fabric.