I use a thin 70% cotton / 30% poly batting for lap quilts. I wash it on delicate in cold water and line dry it (we don't have a tumble dryer). It shrinks a bit, and comes out nice and soft and pliable. I pre-wash all my fabrics and my batting, then I wash the quilt again once it's finished. I like quilts to be nice and soft and cuddly when I gift them. I don't know if 100% cotton batting washes as well. Your quilt will probably come out "crinkly" if you don't wash it at all, though.
Jan