Your first priority is healing and when you have nothing to do you can help with dps. At least this is what the Novice healer tutorial tells you.
Of corse getting at higher levels you will have more experience to know when to heal and when to dps.

Good tank and party will give you minimum healing time and more dps time. If you still don't take that time to dps and just follow the party around then you are getting carried by the party.

Bad party on the other hand, you'll be healing most of the time and much less time to dps. In this case you are the one carrying the party.