Technically, from what I have read, the nutrition on foods - IE MEATS - is pre-cooked, hence would need to be weighed before cooking. However, to me, this isn't practical. I pre cook meats in large volume, prep food daily and therefore weigh my meats cooked. I use a ratio method. I weigh out 6oz cooked chicken, but record it as 8oz. I think the MOST important factor is, whatever you do, just do it consistently. Weigh your foods the same all the time to eliminate variables. Your numbers may be inaccurate, but at least they will be consistently inaccurate. If that makes sense. Other examples below:
Oatmeal - Measure Precooked
Rice - Buy in the bag so its already portioned
Potatoes - Weigh cooked