I made the same quilt that is in your lovely avatar! I made it out of Quilt shop quality flannels - those really great ones that look like men's herringbone suits. I washed and dried the flannel on hot TWICE, as I was advised it would shrink a lot. It went from 42.5 wide to 39 wide!!
I have always been an advocate of pre-washing, but that experience really sealed the deal for me.