Yeah - you only need to worry if you have a genuine leather, suspended saddle (the leather is actually structural - not just a covering for a plastic or metal structure) - they can stretch/shrink/warp otherwise do bad things if allowed to be soaked. Up here in the lovely PNW, I regularly ride in everything from a thick vapor to a raging downpour... never had a problem with any of my saddles and I don't ever bother to cover them.

+1 to worrying about the other parts of the bike.... you'd be amazed at how fast a chain can rust. My bike gets a rinse off, wipe down and re-lube every time I've been riding in the wet.