I don't. I do, however, test suspicious fabrics for bleeding. (I cut a small piece off and drop it in water to see if the dye bleeds into the water, then rub the damp fabric against a piece of white fabric to see if any dye transfers.) If I find a bleeder, I wash that piece of fabric in Retayne to permanently set the color. It is very rare that I find a fabric that needs to be treated, but I am extra-cautious with reds.
Unless making a rag quilt, it's a good idea to prewash and machine dry flannel because it shrinks *so* much.