I straighten out as I go, sort of using the wiggle method. After I while, you get a feel for if the fabric is straight, even if I don't hold it up in the air. I used to do the two ruler thing to get a straight edge, but then saw a Fons and Porter thing where they just cut the first strip oversized and trim the uneven strip in a second step. Then the fabric edge is straight. I agree that you need to check every 18 inches and might have to realign the fabric.