I've heard that you are supposed to protect the clockspring or something in the steering column... but I never did. All last summer I had my top and doors off, even in heavy rain, the carpets and seats have always dried out very fast and never mildewed or started to stink. the only thing was the speakers would hiss now and then until they dried, but they still work fine.
I have ruined books that I left in there, and once I got caught in a storm, even though it was summer, that almost made me hypothermic. I was shivering so badly I thought I would crash, and I could hardly see through the water on the inside of the windshield. Thankfully I wasnt too far from home.