Well.... My 2 wedding quilts from 1984 were both sewn with polyester thread, and it cut through the fabric at the seams, in less than 10 years.
Also, if you're pressing with a cotton setting on your iron, I think it would melt the polyester. Check where the polyester setting is on your iron, compared to where the cotton setting is on your iron. I know if I let my iron tough minky for very long, it will melt it.
I have a couple friends who have used Maxilock serger thread and have pressed it using a cotton setting on their iron, and they lifted up the block and pulled on it, and the thread had indeed melted. The block fell apart.
Sorry, I don't follow what everyone else says, I use my own experience and knowledge. I choose to use cotton thread to piece cotton fabrics.