Dungeons: About 50%, less if I know they're not going to take too much spike damage.

Raids/Savage: I don't heal unless the next raid wide or tank buster will kill someone.
Eden 3 savage is a good example. After breaking the side platforms and using his AOEs, Leviathan will continue auto attacking the tanks until he leaves for his divebombs. The tank can go as low as 20k hp, or about 15% without me having to heal them.