When I join as DPS, as long as the other dps don’t screw up big I always comp them. From my experience they are the least appreciate role.
I can play Tank or Heal role with mediocre and still receive tons of comps, while doing exceptional in dps and get nothing.

When I play tank or healer I usually comp healer or tank, as their performance are more noticeable when I play the respective role but aways comp another dps (as long as no big screw up and not rude) when i play dps