Is it necessary to have Service done on a working microwave oven?

You're very lucky! The best thing you can do, or probably have already done, is to keep it clean, especially inside. The waveguide cover, usually on one wall (something that looks and feels like glazed, stiffish cardboard) can get covered in burnt-on food and can need replacing after a lot of use. Regular wiping down maximises its life.
There's very little to wear out. Perhaps the odd bulb but not really much else.

Colin
January 2010