When doing rag quilts I have never prewashed the flannel. I put my sandwich squares together, stitch an X across them to hold everything together, or quilt them , assemble, clip then I take it outside & give it a really good shake ( the birds collect the threads/ fluff. If it’s a big quilt I take it to the laundromat & use their largest machine, if it’s not so big I launder it at home. After the first spin cycle I stop the machine, take it out & go outside to shake the dickens out of it again- I clean out any stuff left in the washer put it back in and finish the cycle. I do it all again ( outside to shake etc) before it goes into the dryer, after about 20 minutes I take it out, empty the full lint trap, take it out for a good shake then finish drying. I’ve never had any come out bad due to uneven shrinking or anything else. The batting and stitching helps. My family loves raggy quilts.