I prewash every piece of fabric that comes into my house.
At considerable expense, we had square dance outfits made by a professional seamstress but the group that was in charge did not prewash the cotton fabric before taking it to her. Brought them home, washed the outfits and they shrunk. I had to let them out as much as was possible from the seam allowances. Lots of unhappy square dancers. From then on, I made our outfits myself.