I did not opt for any undercoating at the time of purchase.
I recently took a good look under the car, the best I could w/o a lift or ramp, to see how much or how little factor rustproofing had been carried out. I'm no mechanic but I could immediately tell that a very good amount of factory work has been done to prevent rust. For example, you will notice that the wheel wells are coated, in their entirety, with a rhino liner / rubberized - type coating. Looking under, I see that the main frame-type bars and suspension springs are fully and thickly painted with some glossy-looking black paint, looks kind of like the rustoleum black gloss paints. I also a fair amount of liner/rubberized coating throughout the bottom structures on what I would guess is the fuel tank(?) and some other places. Bravo Nissan
Just about the only place where I see exposed metal that would be prone are a few bolts which are exposed shiny steel, and the exhaust, which are completely 'naked' and I'm sure will eventually rust, there already is a small sliver of thin surface rust forming on the exhaust.
I feel quite confident that the Nissan engineers have put some effort into preventing rust, and that with regular washing of the undercarriage during the road salt months, rust should not be a problem. Not too concerned about the exhaust, I think I heard before that the exhaust is not supposed to be coated/rustproofed. Just to be extra safe, next time I get my car washed and dried, I may crawl under and spray some aerosol Rustoleum black over the bare bolts and other areas that look relatively less protected. I am unsure what to do about the exhaust but all in all it is a replaceable part.
Also, for what it's worth, many of the car washes around here (Cambridge/Boston) include as part of the undercarriage wash, a 'rust inhibitor' treatment, although I have no idea what that is.