I have no doubt that rust on the underside of the valve covers happens. I saw it all the time in the late-60s/early-70s when I often worked on old 216/235 engines. Yes, good ventilation and warm-up is key to minimal or no rust. However, I often had to clean out gummed-up "ventilated" filler caps; and, I often had to give up on older engines that had gummed-up/clogged oil lines leading to the rocker arm assembly.
