I use preshrunk Hobbs 80/20 or Quilter's Dream poly. I've never had a poly batting of any kind shrink. I have one table runner that the binding shrank up and now it won't lay flat. I am pretty sure that binding is a Wal-Mart cotton broadcloth since it's a solid and that's the only place I would have gotten a solid back when I made that table runner.
To preshrink batting, put it in the washer and fill with hot water. Do NOT let it agitate!!! Turn the washer to spin and then put it in the dryer.