Alright, not counting criticals in this, because Power Attack doesn't really affect that side of things (+3 damage is +3 damage, critical or not)
But....
If damage after PA is more than damage before PA, then PA, otherwise, do not. The math for when to PA when not oathed doesn't change... it's a fairly simple formula of when to and when not to.
But, when you Oath.... the math gets -very- complex.
If your probabilty to hit (before Oath) is p, and the average damage is d, then your DPR is:
pd before oath
pd+(1-p)pd=
pd+pd-p^2d=
2pd+p^2d
At heroic, you're exchanging 10% to hit for 3 points of damage.
So, if 2(p-0.1)(d+3)+[(p-0.1)^2](d+3) > 2pd+[p^2]d then PA is good.
(2p-0.2)(d+3)+[p^2-0.2p+0.02](d+3) > 2pd+[p^2]d
2pd - 0.2d + 6p - 0.6 + [p^2]d - 0.2pd + 0.02d + 3p^2 - 0.6p + 0.06 > 2pd + [p^2]d
6p - 0.6p - 0.6 + 0.06 - 0.2pd + 0.02d - 0.2d + 3p^2 > 0
5.4p - 0.54 - 0.2pd - 0.18d + 3p^2 > 0
5.4p + 3p^2 > 0.18d + 0.54 + pd/50
Egad this formula is a headache. But take your chance to hit, plug it in, your average damage per hit (which may or may not include Censure), and work it. Use an Excel spread sheet.
So, let's say I have a dwarven persuing avenger with 18 Wis, and 16 Dex. I'm using a mordenkrad, with DWT, at level 1. Average damage per hit is (2d5+2)+4+2= 14
Without Censure:
7 TRUE
8 TRUE
9 TRUE
10 TRUE
11 FALSE
12 FALSE
13 FALSE
So, if I can hit on a 10 without power attack, it is good to power attack.
However with Censure, my average damage per hit goes up to 18:
4 TRUE
5 TRUE
6 TRUE
7 TRUE
8 FALSE
9 FALSE
10 FALSE
11 FALSE
12 FALSE
13 FALSE
So... in this case, it is not a good idea to power attack unless I can hit on a 7 (or more favorably)
Power attack gets worse the more damage you do, but for avengers, the math changes a bit. If you're using a fullblade, however, your average damage is only 10.5 for this character before Censure:
9 TRUE
10 TRUE
11 TRUE
12 TRUE
13 FALSE
14 FALSE
15 FALSE
So use it if you can hit with a 12.
Hope that helps.
EDIT: Working on a version that works across tiers.