I just got a notice for my pass which expires May 4th. I says "you'll save $35 if you renew by your Anniversay Date, May 4, 2003!". It used to be they gave you 30 days after to still get the renewal rate, I've heard some cm's will do it for you but it is still backdated to the anniversary.
If you are not sure you will be going, I would not do a renewal. You save $35 but are taking a chance because it will be effective on the renewal date, not the date you finally go. So you'll have a pass that is already ticking away on the year.