I don't pre-wash my fabric and I don't use steam in my iron when piecing. So I don't know if my opinion counts.
When I've taught classes, my students will steam their fabrics or spritz them with water and iron, and the fabric always gets warped. I just prefer a dry iron.
However, if I know a fabric will shrink, I'll go ahead and pre-wash and dry it with heat. That's for flannel and duck cloth and clothing fabrics. I don't worry about quilting cottons.