In my opinion, yearly maintenance is unnecessary as long as you do a reasonable job of keeping your machine clean (and older machines oiled). If I sewed for 8 hours a day 5 days a week, I'd probably take my machine in for servicing every year or every other year.
I used to take my Bernina in every year, but honestly could tell no difference in it. The last time I took it, when I picked it up I realized I had left the upper tension on a lowered setting because I had been working with monofilament thread. If the tech checked out the stitch at all, he must have been satisfied with the upper tension not being at the factory setting. At that point I saw no reason to pay $100 and be without my machine for a week for cleaning that I could do myself at home.
I'd say a checkup every 10 years is plenty for most home-use machines.