As others have said : don't worry as long as your bike doesn't 'live' outside; make sure to wipe it off after a rainy ride and you'll be fine (I should know, it rains a lot in Belgium and I too commute all year round).
The one thing too avoid is buying secondhand bikes with obvious signs of having lived outside (rusty chain, chainrings, freewheel, spokes, etc.) - you just KNOW you're buying trouble in that case. Anyway, a bit of surface rust here and there really won't make your frame suddenly disintegrate!

Joseph