Be sure to lower your iron heat so the fabric doesn't shrink. My grandma has a couple polyester mixes from back in the day in her stash and she says she always knows when she accidentally gets one because it shrinks when she irons it. It happened to one of my corner squares when we were putting the border on my first quilt. Fortunately we were using 1/2 inch instead if 1/4 inch for seams (so impatient niece had play room) so we were able to fudge it straight again.