Washing before quilting and washing after quilting makes a difference in shrinkage. If fabric is closely quilted to a batt, the amount of shrinkage will be determined by the batting instead of the fabric.
Harriet Hargrave made a quilt to prove this. She used unwashed *flannel* in a quilt, then machine quilted it and washed it. The flannel did not shrink as I would have expected; she had used a batting with max 3% shrinkage (I think it was Hobbs 80/20).
Fabric shrinkage will be much more noticeable in quilts that are tied or do not have quilting lines close together.
I do not prewash fabrics and use Mountain Mist Blue Ribbon 100% cotton batting, which is a very traditional batting. My quilts come out looking like antique quilts -- evenly crinkled because the batting shrinks, but not distorted by uneven or excessive fabric shrinkage.
I'm thinking that the true test of fabric shrinkage in a quilt is washing after quilting rather than before quilting.