different cottons shrink at different percents. This is one of the reasons I wash first. My main reason though is because this is how my dear Granny taught me, and she was an absolute expert.
I baste the short sides together on each cut of fabric, I don't use detergent, and put them through the rinse cycle once. Most shrinkage takes place in the hot dryer; the 'pounding' causes most of the shrinkage. Now I am confident that all the fabric has shrunk and won't pull away from the seams after it is all finished.