Am I being charged for international roaming if my iPhone is powered off?

Impossible. When an iPhone is powered down, it is truly off. All radios, cellular and wifi, are completely powered off and incapable of connecting, so you cannot possible be connecting to any cellular network at all.

When overseas, you can also enable airplane mode, then go and turn wifi only back on. That leaves the cellular radio off, but allows you to still use free wifi hotspots when you are within range of one. Again, since the cellular radio is off, you cannot even connect to any cellular network.

If off, I don't see how you would get charged for international roaming. The phone wouldn't be able to physically know where it was and would think it was in its last known location. Either way, you would only get charged for international roaming if you were using it, ie making a call, getting email, sending messages, etc. Be careful though, if you have apps or email or something that data is automatically set up to "push" to, you will get charged for it.