This isn't an issue in my experience.

Source: I have 10,674 commendations.

DPS are less likely to get them and this is well known. They are commended if they do a good job if a player remembers, but often in the final moment, all memory of the duty gets forgotten and the commendation just defaults to a tank or healer for most people. For myself though, I often try to stand out as a DPS so I usually get commendations on them.