It depends on whether you are talking about piecing, quilting, or embroidery. For piecing and quilting, I like to use the same thread top and bobbin. Some people like to use a finer thread in the bobbin when quilting. For embroidery, you generally want to use a very lightweight thread in the bobbin to avoid too much thread buildup. What I have on hand for that is actually called bobbin thread and is quite fine. Mine is polyester (which is generally stronger than cotton in the same weight).