I change my tension all the time. I start at 0 (no tension) and go up 1/2 a number until I get a balanced stitch. I seldom change my bobbin tension because I use the same bobbin thread for all my piecing. Every repairman I have asked said they use poly thread to set the tension when doing maintenance. New machines are set for poly thread so if you don't use poly you need to find a new normal.