I just had mine done by Krown. There are a lot of places on the underbody that have sharp edges, and paint generally doesn't form a good film on sharp edges. I can take an ohmmeter and slide it along an edge (e.g. along the edges of all the holes that they have in various places) and it's somewhat electrically conductive to the body. So if electrons can get to the metal, the salt water probably can, too. I figure an added layer of oil will be helpful for this.
One thing that the undercoating places do is drill holes for access. They have to use "something" on the holes when putting in the hole plugs (they don't paint the newly-drilled holes, no time for that). Krown (at least the one I went to in Niagara Falls) uses white lithium grease for this. So once these holes are drilled, I think you're kind of committed to having it resprayed each year so that the grease gets refreshed. I did also happen to talk to someone else in Rochester who takes their cars to Krown, and they've never had a problem with the holes rusting.
The rear wheel wells have a carpet-like material as a liner (like trunk carpeting - it doesn't really have any pile, and it's molded into shape). There are lots of other cars besides Altima that are doing this nowadays. When choosing where to have my rustproofing done, I asked the local Ziebart about this. They were unwilling to take the carpet liners off, and I wouldn't want them spraying their tar-like substance on the outside of the carpet, so no Ziebart for me (for this and other reasons). An oil spray like Krown shouldn't be a problem with this. Krown doesn't take the carpets off either. Ideally you want the oil behind the carpet. I'm not sure how much oil they actually got behind it on my car, but I think it's possible to get their sprayer behind the carpet if they go from underneath.
Finally, you might want to look on the underbody of your car near the rear support points, just in front of the rear wheels. A mirror and light will be good enough for taking a look there. Mine had places that looked like they didn't get paint; my best guess is that when the car body goes through the paint line at the factory, it's supported at 4 points, and afterward, they never went back and put paint on the rear two support points of my car. If yours has these bare spots, you might want to go to the dealer to have them painted before any undercoating. I just went ahead and primed/painted these myself.
I also have a posting about this in the other forum