I used to swear by undercoating when I lived in MI and the roads were salted. Living in the desert and my last X I had for 5 years, had no issues, so I wouldn't use it now. If I did live where they salted I would consider some sort of extra protection or I would keep it clean after every salting.
The new models with galvanized sheet metal and all supposedly don't need it.
And the issues PacoPico raised about trapping moisture would worry me a bit.