I'm wary of rubberized coatings because of the horror stories I've heard if they're not absolutely expertly applied. Everything from chipping/flaking to actually accelerating the corrosion process.I doubt it will void anything. If you're in the US, they have to show how a modification would have caused a failure in order to void the warranty on a specific part.
That said, I don't know that wax- or oil-based undercoating make a huge difference. They just eventually wash away. You'd be better off with a properly applied rubberized product from 3M or Rustoleum.
I hope on going to the body shop sometime this week. I believe they're offering a rustcheck type product but I'll need to make sure.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.
Northeast PA.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.