As someone who has mained WHM and SCH for almost a year, I always DPS when I can. And I ususually can in any dungeon, sometimes even in some EX primals.

As a WHM, I love to Divine Seal then Regen and Med II the tank, and Cleric Stance, Presence of Mind and Holy if there are 3+ enemies. If there are only a few, I cleric and Stone II, Aero and Aero II.

As a SCH, I love to Adlo the tank, then Cleric and Shadow Flare, Bio II, Miasma and Bio then Bane, even throwing in some Ruins before I switch back to heal.

When I go as an actual DPS, if I see the healer standing around doing absolutely nothing during times of low-to-no healing, they will not get commendations from me.