I try to get my machines in every year whether they need it or not...they always come back in much nicer running condition...more buttery feel, smoother. But in reality I generally only get organized enough every two to three years, barring any problems. I dust out the bobbin area frequently.
I am fortunate that I have an excellent local and independent machine tech, Eurotech in Vancouver, BC. If you ask around local guilds you can usually find who is the best tech for your brand of machine!