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Mandatory? No, because it depends on how you're configured and what you're trying to accomplish.
If for example every gateway router has equal cost to all destinations, and your not using any type of tracking, i.e. you don't really care which is the actual gateway, no need to set preempt nor have different priorities.
If you prefer one gateway, such as one having a better outbound path, and you're not tracking interfaces, you would set it with a higher priority and also set it with preempt for it reclaim active gateway when it (the router) comes back on-line.
However if one router has better outbound path and you're tracking interface, you'll want to again set it to higher priority (with tracked interface up) and all gateway routers with preempt. This so active gateway router has best current available path.
Even in the latter situation, everything will work without preempt, but not optimally. You could have traffic start with the active gateway router and then need to be redirected to a peer/neighbor router. This is why using preempt isn't really mandatory, but good practice.
