Cover when wet?

[Viirin]

Is it alright to cover the bike when it's wet or is it better to let it dry off?
I cover my bike every night but my friend says you shouldn't cover them when they're wet because of rust damage or something. I was just wondering if i'm slowly damaging my bike.
(It's parked under a tree so it's either this or get the spores and gunge dropping off the bottom of the leaves and that stuff is REALLY hard to get off the tank!)