Generally you only want to use Overpower 2-3 times per pull, but at low levels, adding a fourth at some point can be the difference between keeping aggro and losing it. A good way to balance the TP increase is to have leveled Lancer to grab Invigorate. You won't be able to keep it once you unlock Warrior, but you won't need to spam Overpower as much then. PLD, sadly, only has Flash.