I agree--use what you like! Personally, I try to avoid synthetic materials, so it's all cotton or cotton/bamboo blends for me. But that works for us, because our bedroom never gets nice and cold in the winter the way we like it (though the downstairs is always nice and meat-lockery! :roll: ). A poly batting would be very uncomfortable in our house. I also like my hand-quilting to show, and I feel like the crinkly look of cotton helps highlight it.
So yeah, do what you want! I've even heard of people using old comforters for high-loft batting.