I haven't found polyester thread to be a problem in damaging fabric in quilting but then I've just been using it in my scrap heavy usage quilts and my community quilts. If I were making an heirloon quilt, I would make sure that the thread I used was cotton and matched the fabric. I think it all goes back to the result you are expecting. Match your fabric, thread and batting. Quilts that will be used and washed freqently probably should be made using polyester batting and thread for maximum long life.