When you guys want to know the bad guys AC, is it because you want to know if you hit with out having to ask the DM or is it to calculate the number needed to optimize power attack?

Both actually.
Nobody expects to know the AC of the bad guys until after they have been hit. But it has always been an agreement that the group could convey the AC of any bad guy after it has been struck by one of us.
Once the the first successful hit has been placed, we start to light up the bad guy with the necessary buffs, or reduce those buffs to ensure we do hit it for some damage.
Once the AC is known, it is much easier as players to tell you whether we hit or not and how much damage was amassed.

That being said, it is expected that the DM would play it the same way - not applying all possible buffs knowing the AC, but going off the reasonable expectations of what the bad guys would know or see from the party. But again, once the first hit is made on a party member, then all bets are off and they are free to lay into us with power attacks, deadly aim, etc.

I always want to know simply so I don't have to ask does this hit does this hit, how about this? I don't have power attack at this point so don't really care. Even if I did I wouldn't use it much now that I have 4 attacks.

Power Attack is always 'on' unless I announce otherwise.

Power Attack is always 'on' unless I announce otherwise. Unless I have strong evidence to suggest I wouldn't have a prayer of hitting with a -3 on my attack I always plow in with PA on full throttle.

You either use power attack or you don't. You don't optimize it anymore.

William always uses power attack.

Always.