+1 on all. I have seen the rubber cover "spark plug" type literally full of water a week after a rain storm. I have seen the rubber "slip over the barrel" O-ring type work great, but also seen them crack (always on top, probably because of UV rays) and again, hold water up to the f-connector weeks after a rainstorm.
Best policy is the di-electric grease. If you only need a little, buy those $1 little packs at the auto parts store. They usually try to force you to buy them with any spark plug purchase, so they must be a high profit item...but that way you will only blow a buck or two.
Smear the threads with the gunk before screwing down. it won't hurt to put a little on afterwards, also.