Whether or not you can defer off of the waitlist is school dependent; I know that if you accept a waitlist spot from my med school, you are expected to be at orientation (i.e. you cannot defer).
If you tell them now right out that you are planning on deferring regardless, that will likely hurt your chances of getting an offer off that waitlist. The waitlists are not set in stone, people move up or down based on any updates submitted to the school. An update like that would likely move you down the list.
-admissions committee interviewer / senior medical student