Dont start DPSing really until you are comfortable with your ALL your healing tools (ie. its probably best not to start till you're level 50).
Gauge the damage the tank is receiving - if he is getting knocked down to 75% in a single blow, chances are you are safer not stance dancing and focus on supporting the tank. Also watch how your DPS are - are they moving out of unnecessary damage? Minimising avoidable damage as much as possible? If they are and the tank is not taking a massive hit in HP, stance dance away.
Healer DPS should be viewed as nothing more than a bonus - NO content in FFXIV is tuned with healer DPS in mind.
