i have noticed from the threads on "how neat are you" that many prewash and iron new fabric before using. the only fabrics i have done this with are ones that i am afraid will run like black, red, purple etc. is it really necessary? what if 1/2 of fabric in quilt is prewashed and other pieces are not. will it "shrink" unevenly? I am just curious b/c I have been told both ways - wash and not wash.