I can't think of any situation where a pre pull Regen is optimal. Not from any of the content I participated in anyway.

Let me put it this way. If tanks had the ability to Regen themselves, it would be their go to ability during pulls. When the healer does it, the tank has to compensate. If he's good, he'll take the hate off of you without too much risk to you, but if you have to heal yourself during a pull, something's not right.