I really prefer all cotton fabrics in my quilts, but when it comes to thread I am far less picky. The newer poly threads are not like those fishing line poly threads of our youth! And my LA won't tolerate cotton thread up top.... it snaps every six inches, so I don't even try anymore! I can use cotton thread in the bobbin area, but not on the top. And of course someone is going to say mixing two kinds of thread is a problem, with the poly being stronger than the cotton, but I have quilts that are 20 years old and have been washed numerous times, and show no signs of suffering from that issue, so I don't worry too much about that either. I think you have to decide what works in your world and go with experience. Sometimes the so-called-experts are just flat wrong. (And sometimes we are! LOL!) But I think my quilts will outlive me, at any rate, so I'll just be content with that for now.