Probrably a bit of a late post, but our SOP in my company is to arm the doors and start the demo as soon as the L1 door has been closed! This was due to an APU fire/incident on stand a few years ago with the doors un armed and (I think) a delay in the evacuation!
Makes sense to me! And with regards to outside obstructions... common sense (and SOP!) states "check outside conditions" before you open the door!
S.