If the boss has one thing that will hit you hard (ex: Titan's Mountain Buster, final boss of AK's breath attack, Twintania's Death Sentence, etc.) you should use it directly before it happens (preferably with the SP debuff up). Otherwise, with bosses that just constantly hit you with stuff, you'll want to use it on cooldown. Sometimes it's worth holding onto it for if you drop to half or something and need to mitigate some damage while you're getting topped up to full - but in general you'll just want to hit it when it's up. This applies to trash pulls as well. Just make sure when you use it you won't need it again before you have another 5 stacks of Wrath up, basically.