I took a long-arm quilting class on Saturday, and our instructor told us that the old "synthetic thread will cut your cotton fabric" thing is an old marketing ploy! Way back when, it was really the dyes used in the fabric that would cause most of the problems. She said that she uses a thread called "Isacord" almost exclusively for piecing & for quilting, and it is a poly wrapped poly thread. And what she liked best about it is that is so thin, it cuts down on the bulk when you press your piecing - especially when a bunch of points come together!
For me, I use whatever I have on hand, mostly cotton wrapped poly Coats & Clarks & sometimes "Mercerized" cotton C&C.
Use what works for you!!
sue