I use WD40 all the time. Hasn't hurt anything yet. I don't spray the motor or ESC though. I do it in this order: Air, Water, Air, WD40, a little more Air, and rag to finally dry .
after i wash mine with soad and water in the sink (i pull the electronics out) with a old paint brush i dry it off with the air compressor and then wipe down the plastic arms with WD-40, it really makes them look new and mud doesnt stick as bad if you do it agian with the wd40 at the track.
Just my point of view...WD40 is technically an oil based detergent. It is good for cleaning parts...but for optimum performance you should follow-up with a light coat of machine oil on bearings (the kind that comes in a plastic bottle w/needle . applicator). For CVDs, hinge pins, and pivot balls I use a graphite/teflon powder lubricant. For gears I use moly grease.