I use whatever thread will give the look & effect I want. Normally I use cotton machine quilting thread on the top and So Fine poly prewounds in the bobbin but have quilted with serger thread & even Coats & Clark Dual Duty sewing thread. I think the poly thread cutting through cotton fabric is a myth. Superior Threads has a fact sheet you might want to check out. I know a lot of my cotton & poly cotton clothing is sewn with poly & even nylon thread which gets washed & tugged on way more than any quilt.