A lot is going to hinge on the age of the building you're modeling, and the type of use it's going to represent. Take a look at some prototypes and see what you see. In my experience, these types of structures are relatively new, and the nature of the finish does a good job of preventing rust. It's not like old galvanized corrugated steel, which junks up pretty quickly.
You might see more dust and dirt accumulating around the gutters and kicked up on the sides than you would see rust.
Lee