BEFORE YOU READ THIS: I'm aware that there are many elitists that will dissagree with me however:
It's only been the past decade or so that people began to say that you could 'only use 100% cotton'. Most quilt makers during mom and grandma's eras made them out of anything that was available.
If you're entering it in a contest and it's not an 'art' quilt then maybe it should be only 100% cotton but other than that it really doesn't matter all that much.