Reading this thread from a Canadian perspective where our roads are salted regularly, I am pretty sure any rust protection that comes with a warranty of it own (like Rustcheck or Krown which are known to work well in Canadian winters) would be a good idea. If the manufacturer absconds their duty to the customer because of the application of one or the other, you can turn to the people behind these two products for repair as they come with a good warranty. That said, you need to read the contract carefully as there are probably conditions (like having the car resprayed and/or having inspections at regular intervals). There are lesser known rust treatments, but these two are the big guys in Canada.
By the way, where are you from? It might help reading your messages if you visited your User CP page and filled out more information about yourself.