Pulling the bobbin thread up to the top ensures that you don't get tangled nests on the back; In addition by holding the threads with your fingers whether you are FMQ or piecing ensures that the tension at the beginning of a section or seam is even. Also it makes tying threads and burying them in batting is easier. I do tie off all my threads in the middle of the quilt. If the seam starts at the edge I don't tie because other stitching will cover it.