I'm sitting at 2441 commendations as a PLD main. Honestly there's no rhyme or reason to it. Some runs I'll be freaking awesome and get nothing, and others I'll just phone it in during a roulette and walk away with 6. I think the only time it irks me, is when I join an in-progress with a group of undergeared folk and carry them to victory and they all leave me with nothing. Even them MEH. As tank I nearly always give props to the healer if they kept me alive. Usually I'll only give props to a DD if they actually use Limit Breaks and not let them go to waste which is like almost never.

That said, if you want commendations, do guildhests. By and large the quickest way to farm them.