flannels and homespuns are both good choices for raggy quilts- choose the shirts by feel- if they feel good they are good
don't rule out dresses/jumpers/skirts- you can get alot of nice fabric out of a skirt/jumper!
i generally wash everything in hot water as soon as i get it home from the thrift store- before de-constructing- then cut them up- put together your blocks/raggy quilt- then after clipping wash & dry again.