I do not have a Janome but do have a longarm. I could not sew if I could not change the tension! I had a 4 day class with mine and was told to change the bobbin tension first. I have been doing that with no problems. I check the tension every time I change the bobbin. Only occasionally do I have to change the top tension.
I think this guy you are talking to does not know what he is talking about!