might depend how you PvE. If you are farming mobs and killing one right after the other, and using abilities every GCD for minutes on end then yes you would run out of power. if you are killing one or 2, then running over to the next, i dont think you will run out of power. Thats how i play and never have an issue.
Besides, if you do have a power shortage maybe you can pull pts out of beastmaster to get the talents that boost power regen. I really think its important to put those 31 in champ minimum.