If you run your locos on a regular basis, then modern models like MM's need very little maintenance. Wheel cleaning being the only thing that regularly needs doing.
Most models now have can motors, so the brushes are not accesible and lubriction should be done sparingly (usually be applied with the head of a pin.)
As Warbonnet said; if a loco is running and not making any funny noises, then leave it alone!