Personally, I shop them when I first get them, which means completely taking everything off of the top side of the playfield, cleaning the playfield with magic eraser/alcohol, then novus 2 to polish a bit, then wax. All metal goes through the ultrasonic with a degreaser, then into the vibrating polisher. All plastic (that isnt painted) goes through the ultrasonic with just a general mild cleaner.
Painted plastic gets hand cleaned with a mild cleaner and novus 2/1 to polish them up.
I put it all back together with new rubber everywhere and new balls.
Adjust all the switches in the switch test, fix anything else that doesnt work, and then start playing .
This way, if it starts getting dirty, I know that the dirt is laying down on a layer of wax and will be pretty easy to clean when the time comes, and I can just clean the open areas without taking everything off again.