Unless doing "Bobbin" work I use an ordinary cotton thread in the bobbin but I always use a Schmetz Metallic thread needle,
http://www.schmetzneedles.com/shop/l...php?prod_id=19 someone gave me that tip and I have never had a problem.
I have only just, last week, upgraded my elna 7200, to the elna 720, both excellent machines and in the 7 years I have had the 7200 I have never had to alter the bobbin tension. I have had to do it on some older machines for my pupils and I just mark the original setting before adjusting.