When I have thread left over in my bobbins, I tend to use them when I start attaching the next quilt to the frame. Basting the batting to the backing, then the quilt top to the sandwich and usually the 1st set of basting the sides. Then I'll change over to the color I need for the quilt. In time I'll use up the leftover thread and have an empty bobbin again. When I get down to the last of the spool, I use it for piecing on my regular thread. I hate to waste anything and at the price our bits and pieces for our hobby or business I feel I need to save where ever I can. All my quilts are gifts to others so I try to find a way to save whenever I can.