You absolutely should rustproof every car out there. Even if the factory sprays a rubberized undercoating (many S series saturns had this, for example), there are many other areas which are unprotected and can rust. Saturns rusted in the rear door rocker panels like mad, even though everywhere else was usually very clean. Another typical rust spot is the wheel wells because of all of the stone chips that flying rocks create. These holes in the paint expose the car's metal to the elements and create rust.
I wouldn't waste money on the rubberized undercoating and just go for the oil based stuff as it penerates more places, and its cheaper.
Usually the dealers protection shit is just that.
I do Krown rustproofing on my cars and it works great, and is less than $120cad usually.