I clean mine thoroughly every now and then and keep the ones oiled that need to be. I don't take mine in every year and haven't had problems so far.

I've been told by more than one person in the business that they should be done yearly. It's not just about cleaning but also oiling. If a machine hasn't been used for a long while the oil can go thick so is even more important reason to be serviced.

I have several machines, my Janome 6260and Janome 1600p are used several hours almost daily. I've made it a habit for years to have both tuned up after Christmas EVERY YEAR. Never had a problem with either of them. $40.00 each annually is a small price to pay to keep my workhorses in tip-top shape.
The other machines are cleaned and lubricated regularly by my son or myself.