I would think that its better to do tune ups regulary and not waiting for problems to arrise. Eventhough the engine is still running ok, it most probably is not that efficient and using more fuel than needed. Where only the plugs never changed, or also the other more regular items?
Basically only the plugs have never been changed. I have changed oil at 3,000 intervals and other fluids as service schedule calls for. If my dealers service dept. sees any other items that need attention I have the work done.