i open up everything i can with my older mechanical machines, remove the lint with clean longhandled paintbrushes, wipe clean with a soft cloth. i then turn the wheel, raise and lower foot, etc to check which parts move and oil them.
with my new computerized machines, i just remove the faceplate and clean the feed dogs and bobbin area. a drop of oil on the wick. these machines have quite a few sensors in there, and i wouldnt want to mess them up opening anything...