depends on the game mode and what the game is doing. On HQ/Domination/SnD/anyonther game modes with objectives in the underground bit i always alternate the doors, if my team are capping or planting i close them, if i'm defending i open them.
if it was on it makes exploring the map more easier and more faster. when i 1st played in this map and the doors was closed, i fell from the ladder in that room to down. i had no idea there was "Downstairs" lol. i was like wtf omfg where am i? :p