i have made literally hundreds of flannel quilts- some with pre-washed fabrics - some without-
i can honestly say i've never had a flannel quilt come out ruined- or even damaged in any way- either way-
it will be fine to send it out to be quilted- that part has nothing to do with pre-washing or not-
and when you get it back == bind it then wash it- gently with cool water- it will come out of the dryer fluffy, soft & cozy---and you will love it!
this pic of the flannels quilt i made my sister was made with flannels that were not pre-washed- and the pic is after i washed/dried it- before i gave it to her...