Definately don't power attack
By the way, as a general rule, don't bother power attacking when the thing you are power attacking with does huge amounts of damage per hit.
It's because the average loss damage from a hit turned miss (becuase of the negative on the "to hit" roll is to high) is so great compared to the small potential increase in damage.
The Pit Field is doing 30 points of damage already with a non-magical sword (on average). With criticals and etc. it's probably around 35 on average. Anyway.
The Pit Field also has 7 attacks when using the sword.
Let's say you power attack for 5. 10 extra damage per hit with the sword! Wow! Trouble is, with 4 sword attacks, that -5 is pretty much guaranteed (on average) to turn a hit into a miss. And every time you turn a hit into a miss, you lose out on 35 points of damage. What you gain by power attack (assuming 3 hit and 1 misses) is 30 points of damage extra.
Then of course you need to apply the -5 to the bit/tail/etc. attacks, which don't benefit from the 2x1 power attack bonus, further reducing its benefit (compared to the effect of missing an attack).
To sum: no. Don't power attack at all, even for 1 point. It's just not worth it; you are risking creating a miss, and a miss remove a LOT of damage from the equation.
If you have a magical greatswod, with some nice damage dice or whatever, it's even MORE incentive NOT to power attack at all. Just focus on hitting; that will give you the best average damage per round (against all but crazy high AC's such as AC=50).