Rust Prevention/Undercoating

So, I just bought a 98 Sentra and I love it. It's my first decent car, really, I had my last car since high school and it was my dad's before that. It's in fairly good shape and I want to keep it that way. Given that I live in the snow/salt/rust of Wisconsin, I was looking at getting it rust-proofed which seems to include an undercoating and filling empty metal spaces with yellow goo. Can anyone give me a reason not to go ahead with this, or what to look for when shopping around?