I've found that the all too common trend is to give the commendation to the tank or healer. I play a whm, a blm, a sch, and an archer and my whm gets far more commendations than my blm or bard. For this reason, I tend to ignore the healers and tanks simply because I feel like too many people are commending them simply for doing their job and look at which dps was able to stand out to me because they tend to get overlooked. If nobody stands out, I'll base my decision on other factors like who's name I like the most, lol.