This is a classic question. Bottom line is: it depends on what you're willing to spend, how much spare time you have, and whether you think rustproofing is worth anything at all. You will get people with opinions on the subject that are all over the map.
I did not have mine rustproofed, for a number of reasons, not least of which being I don't think the stuff works. Instead, I try to keep the water drains flowing freely, I painted most of the undercarriage with flat black rustoleum, I touchup the undercarriage whenever I notice a rust spot forming, and I perform regular inspections of the underside during warm weather and oil changes. I've recently begun coating some parts with white lithium grease in an aerosol can (although I recently read that lithium grease is NOT the best anti-oxidant formulation).
Do you have the time, desire, and equipment to keep up with maintenance like that? Do you think rustproofing actually works?
-Ryan