Ah, I think I see why some people are and some people are not seeing spiders. It could be that they do not always appear - their appearance could be based on any combination of sanity level, location and random chance.
Like I've played the game twice now and only once have noticed bugs apparently crawling over my vision (not any surface, just the screen). Now I think about it, they may or may not have been spiders themselves. Either way I found myself brushing my face with my hand, nervously trying to clear them away. Which is just nuts...