It's a common strategy in other MMOs (*cough*WoW*cough*) to pre-apply regen, because tanks generate sufficient enmity to compensate. However, FFXIV is different, and if a healer does it, pulls hate, dies, and then yells at the tank, the tank is fully in the right to tell that healer off, or to leave.

In my experience, Regen is only safe after the first enmity combo is out, and Medica II is only safe once the tank has a really big enmity lead.