Rust proof or not the outside of a steel steerer column beneath the stem
I generally wipe whatever greasy or oily rag I have lying around at the shop on any uncoated steer tubes as a habit. We're in a fairly dry area so it's not of great importance. If I was in a wet area I'd use a product called FrameSaver.

There is no concern with slippage if the steer tube is greasy.