I use what ever thread is the right color for what I am working on- often cotton thread from Connecting Threads- or aurifil when machine piecing. I use silk thread most of the time for hand piecing. but what ever thread I have, my machine likes, is the correct size/weight and color for my project is what I use. the only thing you need to take into consideration when choosing which thread to use is....is it good, strong thread? the weight of the thread sometimes matters (the finer the better for precise stitching= heavy thread means needing narrower seams to make up for the space the thread takes up)