Wash and dry the fabric to get rid of finishes that can distort the grain. Press well, then pull a thread on an uneven end. Trim off the amount on the uneven side of the little space left by pulling the thread. Now you can fold the fabric in fourths and line up the end edge, trim off a smidgeon to make a superclean edge, then measure and cut. Large amounts like backings, I clip and tear.
I have eyeballed the edge and did the wiggle and shake, but it's not as reliable as taking the time to pull a thread, IMO.