You get one and mostly it needs to be 'gone thru' and tuned up. And 'gone thru' doesn't mean you have to clean every switch. Not like an old SS game (or even a newer one) where you might have to replace some or all of the pins in the connectors, replace the display(s), replace a boards(s), battery damage, replacing all the old capacitors etc. If you fix one up and do it correctly, in my experience in home use the darn things almost never break down or even have a failure and only need occasional cleaning and lubing of a few units.

Agree, no real harm if the cleaning is just a piece of paper to wipe out dust from years of sitting, but the switch points have seated themselves together (like a valve getting 'broken in' in an engine) and cleaning an EM switch in the typical sense of filing or burnishing with a tool will remove that. And the black silver oxide that forms on them is not a bad thing, it's a great conductor. That's assuming the points are not actually burnt, but those are the small minority in the machine. A switch that lights lamps won't normally get burnt. A switch in a relay that closes but then another switch on the score motor actually completes the circuit won't normally get burnt.

Eons ago I recall recommendations for light cleaning of EM switches - 'crocus cloth' - but I've never used it or even knew what it was so I looked it up. It's like super fine sandpaper but coated with iron oxide instead of sand abrasive. I assume it works without being harsh but I'd want to wipe the contacts clean afterwards with a piece of paper or a little strip of thin cotton sheet cloth or something to remove any residue.

Or there is the burnishing tool, very very thin blade of metal with a dulled look to it, not abrasive like a file or sandpaper. One ad I saw says it's safe for gold contacts too but no way would I use that on a gold-flashed contact in a solid state game. Strip of paper or business card only for those.

Both things to consider if the switch is adjusted correctly but is still not making good contact but the points are not damaged or worn out or burned from arcing.