I clean the wheels on my locomotives when I can notice the headlight starting to flicker. I run DC only and it is a lot more forgiving than DCC. I have a piece of test track that I use to clean the wheels with a piece of paper towel doused with Alcohol. Dirty wheels come from dirty track. My layout is in a finished basement and with our kids gone, the only reason to go down there is to add salt to the water softner and to go into the train room. Dust is not much of a problem, but I will admit that I really don't have a set cleaning schedule for cleaning the track.
There have been periods when I haven had the chance to run trains for months and when I finally do start I running them, I usually don't have any problems. With the exception of a few switch engines, I usually rotate road engines. I have hidden staging tracks that I store trains that were outbounds that will become inbound trains and usually change them out on a regular basis when I am operating. I try to run them all to keep things lubricated. Perhaps once or twicea year I will go through all of my locomotives and lightly lubricate them. I have a few brass locomotives that are too large for my turntables that live in their box most of the time, and they are also lubricated and let run laps for about an hour to also keep everything running smoothly. Wheel cleaning is part of the normal routine.
I don't like to let any locomotive sit and not run for any long period of time. I don't think it's good to just let them sit idle. When I'm working on the layout, doing scenery or detail work, I'll raise the bridge on the hidden staging track and just let trains run, changing locomotives out after about an hour.
Maybe I have been a bad boy. I don't clean the wheels on my freight equipment. All have metal wheels and I notice very little if any buildup on the wheels.