Everything stated is pretty much true. If you were to undercoat or rustproof it, you'd void your 12 year perforation and corrosion warranty. It's better if you don't spend the $ on doing that - get some sweet rims instead!
I just got my gti and at first I agreed to the rust proofing offered by the dealer. I then decided that its a waste of money and it does void the vw warranty. Also, i read that the aftermarket rust proofing and crack later on and water or slt can get in which will make it rust faster.
Keep your car clean and u are good to go.
But I got my rust proofing from the dealership... that electronic thing that sends a constant current through all metallic parts of the body and prevents rust through electricity... isn't that what all dealers offer?