Part of my tests, Barb, is that I was concerned about melting too. I press every step and use a very hot iron (often all the way up to linen). No problems at all.
The modern poly threads like Glide and So Fine were designed with quilters/cotton fabrics in mind. There are plenty of spools of things in my vintage threads that I would not dare iron or use on cotton fabrics!
I should admit I was also happy using Dual Duty when the standard belief was everything cotton. I can tell you that my quilts from the 70s had no issues from threads sawing through the fibers. The issues were where the fabric itself rotted away in the unquilted areas leaving the stitched/ditch quilted seams strong and firm. So for 20-30 years I mostly used Dual Duty top and bottom. It was only when I started using my friend's long arm that I started using different threads, so 10ish years ago.
The lady I bought the thread from did her own long-arming and her decision was that she was going to use the same thread top and bottom and that all these wonderful cones were surplus to her. It was a win/win for both of us! Because part of it boils down to Bernie gets what Bernie wants.