junebug172 - Actually, no. He might give the order to evacuate, but no American carrier I k ow of has a Captain dictating which doors to use. That's a decision our FAs make.
Apparently your knowledge of all American carriers isn't quite as exhaustive as you thought.
The Captain will order the evacuation including a command that specifies the exits to be used. For example "All Exits" or "Left Side Only" etc. The flight deck crew may have knowledge of a threat or potential threat that the Flight attendants haven't been made aware of. Obviously, the Flight Attendants will then assess their exits.