There is always two sides that speak out on the tearing of fabric. There will be some bruising when tearing fabric but on a positive note, steam pressing the edge brings the threads back together. The selvedge is not necessarily straight but it is usually the straightest edge you will get unless you tear down the length of the fabric, too. Professionals tend to tear the ends since having a straight grain is more crucial to draping than losing a 1/4 "of bruised fabric. The width wise grain is stretchier than the lengthwise grain as I'm sure you've been told by now. Either grain can be used to advantage. Generally the yardage was figured based on a WOF unless stated otherwise in the pattern. Unless it's a one way design it won't make a big difference in each block.