I don't worry too much about the straight of grain thing. I have made dozens of quilts, and they all lay nice and flat, including my borders, without me wasting a bunch of time trying to find exact straight of grain. With today's fabrics it's difficult to find it, and it doesn't really matter! I worry more about getting the pattern of the fabric going straight so that overall the pieces will look like they are cut straight. No one is going to be examining the pieces to see if they are cut on straight of grain...not even judges at quilt shows do that!!!
:-)