Generally speaking, if the game was shopped correctly when sold, apart from a wipe down and wax every 6 months it certainly should be all it needs.

Like Nino I've seen games where this work was 'claimed' to have been done, but simply a quick clean of the easy to reach areas is all thats done.

The cabinet should be clean, each and every mechanism under the playfield cleaned, all inserts (underside) cleaned, lamp boards, subway ramps, ball trough, all above playfield ramps etc.