You mention piecing and quilting. For piecing I use 40 wt. I recently went to a FMQ quilting class and for quilting she explained that she uses 100 wt. silk thread when quilting a tight/dense design. When quilting a less dense design, the 40-50 wt thread should be used. When the design is dense, if the thread is too thick, the detail is not as clear. She also explained that silk thread is very strong because the fibers are longer. I can not afford to change or add to my thread stash any time soon, so I will be using my 40 wt. polyester and cotton for a while.