In general I try to take my little Kenmore in once a year for a check-up, kind of like my cats and my dog. I use my machine a lot, so I try to clean out the fluff in her innards every other bobbin change. I recently did a really thorough oiling -- and it made a big big difference, since I've used it a ton since my last annual check-up! My mechanic is an employee of my LQS, so is not brand specific like a dealership might be. My machine is at least 25 years old (my Mum gave it to me when she stopped using it) and runs like a charm (now that I've oiled it...)
Alison