I almost always (unless the quilt is for a baby or someone in a nursing home) use monofilament thread as my top thread. I do this because I am still not the best at free motion quilting. I use Aurifil thread. It is so great to use that I did not realize, until recently, that it is nylon. I have never had the thread break while sewing. I have never had to adjust the tension. Maybe my Bernina does that for me. At any rate, I am very happy with the Aurifil monofilament thread. I haven't melted it with my iron either. When I am better at FMQ, I'll probably quit using it.