I have a Kenmore that I have had for at least 20 years if not longer and I have had it serviced a couple of times. I oil it and clean it and have only taken it in when it was skipping stitches. I can't tell you how many times I have heard some of my friends say their machine is worse after coming home from a servicing. So for me and my machine, if it's not broken, I don't fix it.
Of course that's just me......not recommending that you should neglect your machines.
)