I was taught to straighten the grain of fabric by pulling the bias of the fabric until the selvages are parallel. When the yardage is very long this is rather daunting to do. However, if you find a "helper" it is very doable. Find the true bias and have you and your helper move down the length of the fabric (pulling in the bias dimension) at the same intervals-maybe 6 inches at a time. Check frequently to be sure you are maintaining the straight grain. I like to do it this way so I can make borders on the lengthwise grain whenever possible. Keep the options open by keeping fabric in one piece. I might add that I like to cut lots of pieces lengthwise rather that crosswise because it is more stable.