If I am running with a well meaning newbie tank, who is still learning (or we lack a tank) I will gladly focus 100% on healing. If I am in unfamiliar content, or running more difficult content I will focus 100% on healing.
If, on the other hand, the tank is really good or the group as a whole is easy to keep up I'll probably DPS. Instances like Esseles I will act primarily as a DPS and heal as we're heading towards the next pull. More than that isn't really necessary, and I want to have fun and not just stand there.
In some cases, when the tank or DPS aren't doing their jobs I may DPS to keep mobs off of myself, or throw out some damage in between heals to help us make an enrage timer.
For the most part, I know what I am capable of, and I know the flashpoints and some of the ops really well. After the first couple pulls I'll have a general idea what the tank and DPS are capable of as well. Once you got that down it's fairly easy to anticipate incoming damage and to heal or DPS accordingly. I couldn't do this when I new, but there is no reason why I can't or shouldn't now. Have a little faith in your healers, often we know what we're doing.