I periodically put a drop of oil in the bottom of the bobbin case holder (the technical name escapes me right now) of my Bernina 440QE, but not each time I change the bobbin. I do take off the needle plate and clean the lint out with a Q-tip every time I change the bobbin. I've been told that there's lint way back there that I can't get to, but as long as it doesn't mess with the thread, bobbin or feed dogs, I'm not going to worry about it too much.