Weighing Chicken?

Depends on what entry you use. There are 'raw' and 'cooked' entries. Raw will be more accurate if you can do that.

We always use the "before" weight.

I think the general rule is to weigh after cooking.
The extra weight in raw meat is lost after cooking and you're not consuming it

Actually, the general rule is to weigh before cooking and use the "raw" entry, it is most accurate. Restaurants also do this with meat.
Its mostly just water weight you lose and different methods can affect how much.

Edited to change weight to weigh.

I would go by the unstarred MFP database raw entry if you can.

The weight will change when cooked, generally loss of water weight I think so the resulting cooked chicken will be higher in calories per gram (or so I am led to understand)

The problem using cooked weights is for say chicken, the longer you cook it the less it will weigh so how do you know your chicken is cooked to the same degree as the one listed?

Again it would be the opposite for boiled vegetables, the longer you cook them the more water they take on the lower the calories per gram gets

lol so does the weight after cooking only apply to pasta? lol

I think if you look at a packet of pasta it gives the nutrition data by dry weight. Again the longer you cook it the more water it absorbs

lol so does the weight after cooking only apply to pasta? lol

No, pasta also should be weighed before. Same idea as the vegetables noted above, the longer it cooked, the more water it absorbed and will have a weight different. Rice as well.

ETA - Haha, 3 at once.

Where it can sometimes get really difficult is when you are cooking rice for 3, you put 50 g per person into the saucepan but then you apportion it by cooked weight.

So in my view the only way to be accurate in this instance is to weigh all of it in its cooked state divide by 3 in this instance and then you can assume 1/3 was the equivalent of 50 g dry

Bit a of B***s Ache but the only way to be sure

Edited to say just using rice as an example, could easily be pasta etc

Wouldn't you weigh it after it's cooked since that's how you're eating it? That seems like a no-brainer to me. I want the weight to reflect what I actually consumed.