When you play online to you tend to go to rooms with the highest jackpost or rooms with less people that may have a lesser jackpot to win? I used to pick the rooms with less people because I just thought that I would stand a better chance of winning. That was not always the case though.
I choose the room with the least people in normally, as I'm not terribly good at it so my chances are perhaps higher. I don't know if it actually works out like that though, as I don't ever seem to win!
I normally go to the least people room because I'm a noob and I don't and I want to learn. Like Green-Moo, which is think like him, I have never won but I have been very close to doing it so I'm keeping my hopes up.
I like to talk to people who are playing so I would choose a room with a lot of people in so it is more sociable and chatty. I know it is difficult to follow bingo when you are chatting, but a lot of the rooms I go in fill out the card automatically so you just have to press bingo when you have it.