Unless you live in a dry, arid region, chances are you should be using a rust inhibitor spray on your vehicle. It isn't just the rain and snow that is causing all the rust either. A big part of the problem is the salt used on roads to melt the snow and ice. As the saltwater sprays up under the vehicle, it coats everything and begins to cause corrosion. Do not be fooled by the fact that body of your car has no rust on it; there can be other damage occurring to important parts of the car. Consider the following when you are debating purchasing a rust inhibitor or not.
You may not notice rust as it gets started. Any bare metal on the vehicle is susceptible to rust. A small scratch that has removed the paint and exposed the metal can be the start of something big. Left alone, rust can grow underneath paint that has bubbled slightly or is beginning to peel. You may be surprised to find that the metal under a fender is all rusted so the fender starts to fall off.
While the body may not be rusting due to the paint and newer materials, parts in the engine compartment and under carriage are not protected. Brake lines, fuel lines, cooling tubes and electrical connectors are susceptible to rusting and corroding due to salt and water. If nothing else, you may want to use the rust inhibitor spray on these parts to keep everything working properly.
If you spent money buying chrome wheels, trim or pipes for your vehicle, you do not want it to look bad after one winter on the roads. Make sure the parts are clean, dry and rust free and then spray with a light coat of rust inhibitor to protect them and keep them looking good.
Car manufacturers have worked hard to create body parts that do not rust. However, that does not mean that rusting will not occur. Take a bit of time to look the body over, perhaps even removing some pieces to see what is going on underneath them. A light coat of rust inhibitor will not hurt even if everything looks fine. Then, once the body is protected, spray metal parts under the car and any chrome pieces you added. Rust not only looks bad, but can have parts falling off that you need to safely operate the vehicle.