In sewing class, back in the olden days, we learned to pre-wash all fabric. I still do that, although I tend to mostly get it wet with hand-warm water, wring thoroughly, then let the dryer do the shrinking. I once threw a bunch of fat quarters through the wash and they shrank so much I couldn't make the pattern I had bought the kit for. I wouldn't pre-wash any pre-cut fabric again.
If you participate in most swaps, the fabric used in them is to be pre-washed.