I have purchased a whole bolt of bleached and unbleached muslin... it was ..."Premium, Pre-shrunk" and paid top $$. Guess what I cut 36 inches washed and ... 33 inches came out of the dryer.
So .. yes I preshrink ... I have even learned never to trust the label.
I have had fabric shrink right on the ironing board .. a little heat ... and some steam ... and a 3 inch cut was no longer a 3inch cut.... so I wash and dry.