i use flannel all the time for quilt backs...pre-wash your flannel toss it in the dryer, then press well it will be just fine...cozy soft and snuggly.
some people seem to really stress over using flannels...i've used flannels in quilts since the early 80's and have never had a problem with shrinkage, stretching, or it not holding up....flannel is wonderful to work with....that being said. i do buy good double sided flannels, not light weight single sided thin (stuff)
i only wash it once, in a normal wash and i toss it into a regular dryer. flannel is a looser weave than regular quilting cottons and the edges will fray more than other fabrics, you can either use a pinking blade or pinking shears to trim the edges that will cut down on the amount of fraying. or you can put it into a mesh bag to launder....i never worry about it , i just toss it in...then when it''s all done i trim the edges and get rid of all the strings...but the strings do stress out some people when they open their dryers..