No, I don't think you should particularly feel bad about commending DPS. As a DPS main, when I run into a BRD that I notice is doing a good job, they're always going to get my comm. As for other instances, it really depends on who leaves a lasting impression on me. For example, last night while I was leveling SCH, I gave mostly the DPS comms because they were actively AOEing down mobs for large pulls (I prioritized melee in this instance just because it seems to be a rare occasion to see a melee DPS actually using their AOE skills). I did give a few tanks comms just for saying things like "This is my first time tanking this instance, so be patient, please!" or "Do you want large or small pulls, Hyomin?" Showing consideration is very important, and I felt that they should know that.