Caps you can visually check really. when the foam raises close to or past the rubber surround, time to chuck them. what can happen is the foam acts like a wick a drains the cartdridge into the waste tank. caps tend to last 8 months to a year, but can go longer or shorter.
Dampers are a different story, i tend to run them until they fail. sometimes a working damper can break if it is taken off, say to change one behind it. one of 2 things fail here, either the mesh screen "filter" gets plugged with ink sediment, or more commonly, the seal that sits on the head manifold dies.
Optical eyes, like the cropmark reader i only clean them when they stop reading. encoder strip is touchy, needs to be cleaned when it starts stair-stepping the prints, but excessive cleaning can scuff it, rendering it useless.
Other things to keep clean - the rail the carriage bearings ride on. ink dust and other crud tends to settle on. clean it off a few times a year, and give it a good wiping with some 3 in 1 oil. should keep it nice and quiet.
