It depends on each school. For instance, Yale tolds its wailistees that they would only honor requests for deferral for students taken off of the waitlist by June 30. I assume most schools have similar policies.
I would recommend asking if a "pre-approved" deferral is possible. That way, if they take you off of the waitlist, they know immediately that you want to defer and can go to the next person automatically. Good luck.