I think it can only help to prevent the onset of rust on your car (oil based stuff like krown or rust check). I still think it is a good idea if you can afford it. It will help your car last longer if you keep it and improve the resale if you decide to sell it. Trust me, trying to sell a 11 yr old minivan with rotted out rocker panels doesn't get you much. I could have doubled my selling price if the body was in good shape. I don't trust the dealer either. They will try to sell you their rust proofing, ours did but she didn't push it when I told her I wasn't interested. Also, at Krown if you rust proof before the car is 1 yr old and keep up with the annual applications they have a garrantee.
2009 Pontiac Vibe - 1.8L 5-speed 1SB Carbon Gray Metallic with 20% tint