OK; Rust-proofing/Undercoating?

Well, a couple posters on another forum mentioned that if bare metal starts to rust the paint will peel and bubble and you can simply take care of it, while rust can work hidden from view underneath undercoating. I would hesitantly tend to agree, although if not driven in snow or salt I think it might take decades for that to happen. Just wanna do this right, and NEVER HAVE TO DO IT AGAIN!