"What does it say to do if the temps are below 300?"
Depends, my present airlines 320 says "as required", a previous airlines says "brakes on".
"releasing brakes after chocks ON helps cool them faster"
True but I am not sure faster cooling is the aim. Even cooling is more important for brake life and is achieved with the park brake on.
I cant find a reference for this other some guidance from an airline several jobs ago.