The nutrition facts you see on most chicken is for 4 ounces raw. Perdue has the nutrition facts for it cooked. a serving is 3 ounces versus 4 ounces.
I personally weigh mine cooked because it shrinks due to the water loss. However, some people will argue that if you measure it cooked, you are getting more than what the nutrition facts are. I would keep it simple and just weight meats either raw or cooked and stick with that method. Once you stick with a method, you will be able to consistently track it and establish if you need more or less depending on your goals.
The only things I measure raw are items that I have to add water to in order to cook them. For example, rice, oatmeal cream of wheat, pasta, etc.