Unless you live near a body of salt water, humidity isn't really an issue. The only metal used in computers, that I'm aware of anyway) that suffers from corrosion readily from atmospheric conditions is steel. Gold (used on contacts), copper, aluminum, and nickel (used to plate heat sinks and whatnot) are either non-reactive to most common substances or they form a really thin surface layer of corrosion that protects the rest of the metal from further corrosion.
Solder is a different story since it's an almagamation of metals, and I'm too lazy to see how good it is against atmospheric corrosion.
EDIT: Also I lived on an island that regularly gets 70-80% humidity in the summer, and I had a computer that worked just fine over several years. The only thing that showed signs of corrosion was the case.