Yeah it's better to weigh ur food raw and plug it in to ur macro counter raw. Chicken and other meats will lose water and some fat (depending on the cut of meat) but the protein will be the same. Chicken doesn't have much fat so I wouldn't be too concerned with losing a gram or 2 per pound.
Same concept with things like oatmeal and pasta which absorb water during cooking.
Since there's no real uniform way to determine how much water chicken is losing (because u may cook it for 5 mins or 10 or 20) or how much water pasta and oats are absorbing it's easier just to use the raw measurements.
There are apps that will estimate values for cooked foods but due to loss/gain fluids the volume isn't gonna always be uniform. However, 16oz raw chicken is always gonna be the same...
