Do what I did to test a poly thread. I sewed with it, then took it to the ironing board to see if I could melt it with my iron. I couldn't, and my iron gets very hot. However, there are different qualities of polyester thread; you really need to test each one yourself to be sure.
There was a post on the QB awhile ago from someone who had used both cotton thread and silk thread for a hand appliqued quilt. She said that 7 years later she was having to repair the pieces that had been sewn on with the silk thread; the pieces that had been sewn on with cotton thread were fine. That was just one post, but it does give pause.....