I haven't used the Premo-soft, but I have used polyester thread for piecing. What I did was lay a few strands on some fabric and iron it repeatedly on the hottest setting of my iron. Nothing happened to it, so I figured it would be fine for piecing. Polyester is made in different ways, so I don't know if all polyester thread can take that much heat, but I know it does have a fairly high tolerance for heat -- more than nylon thread.