Agree -- heavy starch the fabric before you cut. A half-and-half solution of Sta-Flo liquid laundry starch and water works well. I "paint" this solution onto yardage, allow a couple of minutes for the fibers to absorb the starch, then toss in dryer and iron with steam. Starch stabilizes the fabric so bias edges are less likely to distort from handling; however, you still need to be careful with how you handle those edges. I would starch the sashing fabric as well as the fabric that will be cut into diamonds. Incidentally, starch also helps with machine quilting to prevent unwanted tucks and puckers, especially if you starch the backing fabric as well as the top.