Jerry,
My quick response would be like many - that rustproofing is not needed any longer, and that it might even be detrimental in that it might create small areas that actually collect salt from road treatments.
But, after doing quick skim of various bits online, it appears some folks still support this practice - but, then an added complication arises - whether you mean/want undercoating in addition to rustproofing?
Hmmm ... me? I would forgo both, but work to keep the wheel wells, and underside sprayed off, weather permitting - with a truck this is easier due to easier access.
And, IMMEDIATELY touch up any paint chips! Even on the lower panels!
I lived in coastal Maine for five years in the 70-80s, and know the devastation rust can cause - but, based on my knowledge of current manufacturing methods, and coatings, I'm in the "no longer required," category.