It's a good idea to get your undercarriage cleaned about every month, and even more frequently in the winter. Just take it through an automated car wash and spring for the 'undercarraige spray'. This will remove most of the caked on salt and dirt that can promote rust and rot. Unless your spending big money on a stainless steel exhaust system, it will rust out if not maintained. Although your muffler warranty may cover that expense, the labor and other materials will cost you. Better to spend a little money each winter then have to spring for the whole thing!