Dina, there are lots of quilts with mixed fabrics so it's completely up to you.
I did this though with one of my early quilts, and found that the quilt behaves quite differently than all cotton - it's hot hot hot - doesn't breathe the same at all for warmer weather, but it also doesn't keep you warm in the winter.
For me it would depend on how it's going to be used - lots of charity quilts have tons of mixed fibers, going to the homeless or out with trauma / emergency workers; the poly adds some good sturdiness.