I think the quality of things have changed but My mother in law made quilt for my daughter and used poly thread and in the multiple washings the thread cut the fibers in the cotton fabric and her quilt was ruined. I would think if the quilt is going to be used and washed a lot it would be a good idea to match fiber contents. But in the end it is your choice. Others may not have had the same experience I have had.
Lynda