As I understand it, the way Anime Expo seem to do it is that if there's less people in the video room, they would do subbed, when there's a full pack of people, they seem to go with dub (probably for the reason that alot of people on the back will have a hard time reading the subtitles as it is 1 - too far away, subtitles become too small to read, 2 - people sitting in the front will block the subtitles (specially the tall guys).
I think we can adopt the same approach unless the anime being played doesn't have dub at all.
