From what little research I've done on the internet, it seems there's a general consensus to simply be consistent. If you measure it raw, measure it raw every time.
I suppose it might be slightly more accurate to measure raw since water retention during cooking is variable. I'm still skeptical though.
An interesting point was brought up: if you buy 12 oz of raw chicken and divide into three relatively even chunks, each chunk probably isn't going to be 4 oz, but if you ate the whole package over a day, you would have ended up eating 12 oz anyway. So, there's a bit of consolation.
Personally, and I ain't even close to an expert on this, I would measure raw and leave it at that. I hope that helps.