If you are playing a healer and get ZERO, every time, you are doing something wrong.
If you are playing a tank and get ZERO, every time, you are doing something wrong.
If you are playing a DPS and get ZERO, every time, you are playing a DPS, don't expect them.

When playing Healer, you will generally reward the Tank, because that's who you're paying attention to. If the tank sucks, or the run is just going awful, you'd probably reward nobody.
When playing a DPS, you are better off rewarding whoever made the run smoother. This will usually be the Healer (since they can rein in the pace that which the Tank pulls things), but there are just as many awful healers as there are awful tanks.

And DPS are generally "awful in general" and you have to be playing your Role (eg BRD's have songs, use them) to get a commendation out of it.

As a healer, I'll usually award the Tank unless the Tank makes the party wipe, or the Tank exits the instance before I get a chance to.

8-person parties are harder determine if anyone deserves it, as a healer your eyes are on the HP bars, and only magic casts show up there, so we have really no idea what the DPS is doing.