I am a new quilter as well but have actually never paid much attention to whether a thread is cotton or poly, only if I'm making something that will be exposed to high amounts of heat (pot holders, rice packs, etc), then I make sure it's cotton. I will say that I have inherited (by marriage) some of those "necessity" quilts and any that have developed holes or rips have NEVER been at the seams. Any that have any signs of wear and tear are either at the edges of the binding (I have one that the binding has ripped where it is folded, as in the very outside edge of the quilt, and it has ripped almost the entire way around the quilt!) or somewhere in the center of a square or the backing. Not one that is showing signs of wear and tear has been around the seams.

Now I have no idea what type of thread was used of course, some I can't even tell for sure what type of fabric it is other than some type of clothing material lol There may be some that wear away at the seams, but so far that has not been my experience. The fabric seems to wear away long before the thread does, and it is completely unrelated to the type of thread because it happens in areas that have not been stitched or quilted nearby.