I just closed my pool with 2 main drains. The main drains were the last I closed....... for no other reason other than I thought they were going to give me problems. I blew out the lines until I saw the air bubbles gurgling at the surface...... then waited a few minutes at which point I "air locked" the mains by closing the main drain control valve. As I understand, this "air lock" will prevent the water from filling the pipe provided there are no pipe/valve leaks.

I feel confident with my pool closing in that I've spoken with friends and co-workers who close their own pools and state they can't even get their mains to gurgle, they just hope they pushed the water in the pipe down below the freeze line, then air lock.