It depends on the machine. The Janome 10001 needs an annual checkup. Can't always tell when things are wrong and since it is not only expensive, but also electronic, I choose to do as the manufacturer recommends. Now that it is 10 years old, out of warranty and had some work done last year (like replacing bobbin casing, etc.), I will probably go to 2 years since I don't have to worry about voiding the warranty. My inexpensive machines only go when they have problems, i.e., never. Not sure what I'll do with the new Janome embroidery machine. To answer what I believe is part of your question, if the machine is worth less than a few hundred to replace, I wouldn't take it in unless I was having problems. Bench charge is too high.
Blessings. Cherie