Undercoating the underside of your vehicle can definately help stop corrision. Another way to help before you spray the anti-rust throughout is to paint the underneath as much as you can. I prefer every two years. All I do is get under there with a can of black rust paint (white if it's white under there)and spray a new layer of paint. If the vehicle is newer, this will help alot. It takes not more than a couple of hours, but I suggest that you use safety goggles. I've been doing this for alot of years and the vehicle's bodies stay mint. Check out The Auto Parts Source site for Great Deals on Parts.
It's pretty easy to stop the rust from spreading if you catch it quick. If you let it spread, it'll spread fast, and in no time there will be a problem. Bodywork can be expensive.