I'm personally going for as much PIE as possible on my WHM. More MP and better MP regen is just better for my playstyle. I don't like to stand around idle in between heals, I'll toss out an Aero, or Stone II, heck sometimes Swiftcast Holy on adds. Sitting there waiting for MP just isn't fun, and with all this high piety armor I've focused on I don't have many mana issues. (usually finish a fight with 1-2k MP left)