I have used poly/cotton in many quilts I've made over the years. Do be aware however that the ironing temps are lower. If it's a quilt for a child, have found that they wash and wear much longer than 100% cotton (don't send the quilt police after me!). Have also found poly/cotton holds up much better than very thin, inexpensive cotton from some of the discount stores. Now...all of that being said most of my quilts are made with good quality cottons. I come from the school of "use what you have".
Brenda