I said toe edge, not either. popping off your heel in a straight air will more then likely throw you off balance, while popping off your toes is much more natural and easier to keep your balance.
popping flat base just doesn't make much sense really, plus i find its easier for your board to pivot by accident when trying to keep it flat causing someone to catch their edge off the jump.
I find this is true with RC boards. They get squirly off the lip when flat based so it's def safer to pop off an edge, and obviously the best edge is toes.
But with camber, flat based straight airs all day.