Great answer! Just an added thought: Wash the sample swatches and dry them the same way you're going to wash and dry the quilt when it's done. I use the heck out of my couch quilts, so they get a warm wash and a pretty hot dryer.
I work in flannel a LOT. I wash it warm and dry it hot TWICE. I've put flannel that's been washed-dried twice on the back of a cotton top that hasn't been washed and it all does the nice little crinkle thing on both sides!
Originally Posted by Tartan
Cut a 4 inch square from your left-over quilt top fabric. When your flannel comes, cut a 4 inch square from it. Wash the two squares and dry them. Trace a four inch square on paper and see how your washed and dryed squares measure up. If they both shrank at the same rate, then go ahead and quilt. If your flannel really shrinks alot more, maybe you will need to pre-wash it.