Cathy, I don't think 1% batt shrinkage is going to be a problem for you.
I don't know about other fibers, but 100% cotton gets softer with every washing. That's one of the reasons why I favor the old-fashioned Mountain Mist Blue Ribbon 100% cotton batting. (I wouldn't recommend it for hand quilting, though; too tough!). If I want higher lofter, then I like Hobbs 80/20. Both of these battings require fairly close quilting (3" or less) and shrink about 3%, giving a crinkled effect.
With poly, I think you need to be careful about brand and quilting distance if it is to be used a lot, as in a child's quilt. I had a bad experience with one that balled up badly over time. If I were to use poly, I'd probably try the Quilter's Dream polydown or something else in the QD product range.