Personally, if I'm caught out in the rain, once my bike has been wiped and dried, I also apply a fresh coat of lube to the chain as well. If you've got caught in "muck," you may also need to clean it, but a decent rain will actually get rid of all that.

Obviously, you don't want to leave your bike in the rain, but water doesn't do as much harm as you think, as long as you wipe it down. It's high-pressure water from a hose that's misdirected and gets into sealed bearings, etc. that causes problems. Generally, a normal rain won't cause any issues.