Originally Posted by feline fanatic
While that is true, polyester threads can melt when ironed at a high temperature, like those used for cotton fabric.
I used polyester thread for my patchwork for many years, and believe that the cotton fabric protects the polyester thread to some degree when pressing (the iron does not sit directly on the polyester thread). Still, it made me wonder whether the polyester might have been affected and I just did not realize it. For that reason, I switched to a 100% cotton thread for patchwork. I do use a high quality cotton thread, as I spent a lot of my time cleaning the lint out of my machine when I used a cheaper thread.
I have not done any actual testing to determine whether this was absolutely necessary or not.
On the other hand, I do use polyester thread for quilting, because I don't iron the completed quilt.