If a tank speedruns well and generally makes the dungeon fast and smooth i'll commend him. If ifrit or garuda go well, i'll commend tank too. Usually commend the healer at titan. I don't commend lvl 50s spamming guild hests and being overly polite trying to root for commendations xD ( god its so obvious...)

dps classes get the fun and easy job, so we don't get commendations as much....

I think commendations are often given out when the party runs into some trouble...and usually its the tank and healer which sorts it ( usually )

However, when u reeeeally think about it, most of the content u use DF for is simple, and when u get someone chatty who brings some life to the party besides the usual farm drones, then yes 100% i'll commend this person.