I honestly don't think it is. I only put 1 point into Mana regen (Just to get something over time) and have max RB PoS. In myth, The dex bonus after a certain point makes PoS's cooldown so low, It should be able to cover all needs as long as you pay attention.
If my MP gets too low, there's always extra large mana pots and extra large rejuvenation pots(which recover mp while using the HP pot cooldown... awesome) that can get me out of a pickle. Plus, you have guild pots. It's totally up to you what you use, though.
Aurum211: Myth Healer 7th RB (350% exp rate RAWR)
Gummygumdrop: Myth Lime 4th RB (on leave for now...)
Mabinogi IGN: Zinc210: total lvl 7xx