Use it as often as possible in the fight, but use it smart. I know personally my trinket will come off cooldown on the Deathwing face fight when a arm tentacle is at about 2-3% health. It's not a good idea to pop a CD when you know you're that close to swapping targets. You want to use it as much uptime as you can get through the fight, not just keeping it on cooldown the longest.