When I did garment construction we used to pull the fabric on the bias to straighten the grain before we made any cuts. With quilting I don't worry about it so much. I wash my fabric, starch and iron the portion that I'm going to use, then make a guestimate cut (if I'm portioning) or if I'm using all of it, I fold it lengthwise until the fold doesn't have any creased and as close to "matching selveges" as possible, then I'll make a straight cut on the right side and that is the side I will work from.