it depends---some fabrics come through the door and straight into the washer-some don't get washed until the quilt is complete-
just like everything else with quilting- every project is it's own- has it's own (feel) (personality) needs and requirements- each is treated as an individual -- some need to be prewashed- some do not --
sometimes i am concerned about a fabric shrinking differently- sometimes the fabric has a funky smell- or feel to it-
sometimes it has (old-set in) creases- washing helps remove them-
if it's flannel i prewash unless it's for a raggy quilt- if it's batik or a hand dye i always prewash-
if its pre-cuts i do not pre-wash. it all depends on the project and fabrics---there is no right or wrong- and no reason for people to get all freaked out about it-
for 30 years i made clothes for my siblings, children, grandchildren- costumes- lots of different items- never once did i pre-wash any fabric i used---i never had an item shrink, run or fall apart because of it...it's only been since quilting that the whole (issue) has come up- before that i never even thought about it.