I agree to check out the reputation of the technicians before taking it in. Years ago (maybe 10+?), when I took my Bernina into an authorized Bernina dealership for an annual check-up and tuning, the cost was over $100 and there was nothing wrong with the machine! I stopped doing this when I realized that the technician had not even changed the upper tension back to 5 (normal factory setting) for the "tune-up". (I had inadvertently left it on a lower setting for a specific project I was working on.) I figured that he had either (1) skipped the tune-up because the machine didn't need it but charged me anyway, or (2) tuned it up with the wrong tension setting.
I do think a professional cleaning and tune-up every once in awhile is a good idea, but most machines that aren't being used 8 hours a day every day simply do not need them every year.