Jeanette...I use a mixture for my quilt tops...... I don't always know what I have for sure as far as the percentage cuz I've had most of my fabrics given to me, but alot of times when I go to use it and it doesn't handle well, I chuck it! Thats when I figure out it must not have much cotton in it.... but it has to have some in it for me to use it on my regular quilting projects. otherwise it does not machine quilt well at all.
I do try to make sure my backings are all cotton tho...unless I am tying the quilt instead of machine quilting it.