It usually depends on what i see from the healer. If the healer goes out of SP alot quicker than it should, i do tend to look at the health of the dps and the tank more.
At that point either the tank is doing something wrong, the dps is getting to much damage or the healer is not using his SP correctly.
Depending on that, someone will get the commendation

But i do have to agree it also depends heavily on role of myself.
When a tank, its alot easier to see wether the offtank is doing a good job. When a dps, you know what you are doing so its easier to judge wether the other dps is doing enough.
Healer i dont play to often.

Have to agree commendations for dps are a good bit more rare than for healer and tank ofcourse for obvious reasons. Less healer/tanks played make them more "valuable" than dps and people want to encourage them by it. Dps is mostly seen as the "easy" task in PvE (mostly coming from experience in other mmorpg's) and ofcourse the type of people youre partied with.