I would actually disagree with this in most scenarios. -Most- bosses are already immune to Stun so you'll want to use OGCD Stuns upon cooldown for the damage that they cause. Honestly, the only exception I can really think of is Ifrit's Eruptions that can be Stunned. There are adds in several boss fights which you need to Stun and don't want to build the resistance up, Leviathan, T5, Forgall (even this can be silenced though if stun resistance is too high), orb phase of Brawler etc. However, it's something that you have to learn on a fight by fight basis. I will tell you that my default mode is to use Brutal Swing (WAR stun) as soon as it is up.
