I don’t cover up my bike. I’ve spent A LOT of time on my bike, pouring a lot of sweat – many multi-hour rides. But you know what? I’ve never seen any adverse issues due to it. Perhaps I’m lucky, perhaps it’s not normal. Either way, I don’t use one.
Elsewhere, in a galaxy far, far away:
Rear derailer cable, bottom bracket area, about one year after replacement. I guess A LOT of time is what Einstein had some theories about.
35 years ago my mother told me to always use protection. It was good advice back then and still is today! Just in case you need an alternative to the helmet discussion.