Batteries - Very important. A new set of batteries is far less cheaper than a new CPU board. Change yearly.
Clean and polish playfield - Depends of how much the game is played. At least once a year.
Ball change - I examine my balls and I usually throw them in the tumbler before discarding them. If they ae still godd, I use them again - in the same machine.
Machine cabinet clean - Mr Sheen once a year and windex on the playfield

Usually, a fault would occur somewhere along the line, and while the playfield is up, I check everything looks ok.

DMDs - When powering on the machine, any errors are reported - so you attend to them as they occur.

EMs - Occasionally check contacts around the playfield and inspect the stepper units