+1 on changing by choice and not because it ran out. I find that the tension changes as the bobbin decreases I size. I just finished a heavy custom and found one block that has issues. May be my backlash spring in the bobbin case needs to be changed. The remainder of the quilt had perfect tension and done by computer, so the speed was consistent. Sometimes, I just forget to check it though. So many little things to remember. I have been trying to do pre-emotive bobbin changes to prevent a thread break in the quilt where it might be noticed.