I never prewash my flannels when making a raggy quilt. I’ve never had a flannel fabric bleed color. I’ve never prewashed precuts. And, I’ve never had a quilt not turn out because of these practices. Flannel raggy quilts come out great. I made one from precut flannel 9” squares that turned out so great.
I only wash a new made raggy quilt once, I clip, take it outside & shake it really good, wash it in a normal cycle. Take it outside again & shake it good then toss it into the dryer. I take it out every 15-20 minutes to shake it again & empty the lint trap until it’s done. Fabulous soft, cozy much loved quilts.