In really low level dungeons (Sastasha, Tamtara, Copperbell) I generally only heal the tank if they get to like 60% when we're against Mobs. I'm a little more strict on my healing during boss fights but still not like "omg their hp is at 50%! They're going to die!". Those dungeons don't have any tank busters so... But I have had ACNs who heal their "friend" the tank when they drop to 80%. I ask them why, and they complain that I'm not. I always point out that there is no reason to heal them so often in low level dungeons and for them to please let me do my job. If the tank dies you can complain otherwise let the healer heal.