Typically I'll be more likely to give my commendation to a DPS instead of the healer. Don't wanna perpetuate the thought of "Only tanks and healers give each other a commendation". If the DPS actually does stuff and holds a conversation with me, I'll give you the commendation.

+ Points for killing in the kill order
+ Points for checking your aggro (slightly better now with Shield Oath)
+ Clicking items and stuff while we're finishing up a group (grabbing lantern oils and using them while we're still fighting so when the mob dies, we can keep moving)
+ Points for actually talking to me during the dungeon, simply saying "HI!" goes a long way.

Now, if the healer is being a badass by battle-raising people and keeping me alive through hard hitting attacks (Everyone's Grudge) then that will help determine who I give it to. Sometimes I have really kickass parties and I have no idea who to give my commendation to, so I just close my eyes and click someone's name at random lol.