One little detail the other poster left out is that your new AP will start when the last one expires, not when you renew. So it does benefit you to upgrade early and avoid any potential price increases that might hit between your 60 day window before expiration and the exp date itself.
Another note if you are buying a Florida resident pass and doing the monthly payment. If you renew fairly early you could have a month or two where you are charged for both the new and old passes.