i make lots of flannel quilts- and always wonder about those (rumors) that it stretches and warps...
i have not had that problem-
purchase good quality-double sided flannel-
if it is thin-limp it may stretch out of shape i suppose
a good quilt shop quality (benartex puts out the most wonderful flannels) is going to behave.
it does help to starch before cutting and use a sharp rotory blade- and new needle- flannels will dull both faster than other cottons.
if you find the flannels you are using do stretch- starch should take care of the problem (and a new blade)