I agree , straight is when the threads are straight and perpendicular to the selvage. However with quilting, I do not worry about it. I prewash and line up selvages and let the cut edges go where ever so the fold has not creasing in it.
Now, if I was making clothing, I would be pulling the fabric to straighten it out. I've pulled backings a bit, but since they aren't cut into small pieces and sew every which way, they can be problematic.