If you are using the controller as a DHCP server, you will need to use the "split scope" strategy, where you serve the first half of the scope on one controller and the second half on another. Why? Because if the first controller goes down, there needs to still be a dhcp server to provide addresses. If you duplicate the same scope on both controllers, clients will end up getting duplicate dhcp addresses, which is a disaster. Even though a backup master cannot terminate access points, it will still answer to dhcp requests if a dhcp server is defined. Long story short, use an external DHCP server to avoid that issue and make the default gateway an external router, so that you do not have state issues with clients that fail over. Using external routing sidesteps the issue of your clients having the default gateway of the first controller, but not being able to go anywhere upon failover.
I hope that helps...