Depends on the situation. Play it safe by being formal for first impressions. Initial contact with a potential client is not appropriate via e-mail unless other means have not been successful.
Formality is still appropriate when encountering someone for the first time.
Once a client is well known, a more casual approach might be appropriate, but never with a new client. This is assuming we do not already have an ongoing, less formal relationship with the client. Just because initial contact begins online does not mean we can forgo respect. That was a great meeting you folks put together, Jack . . . Hi is too informal.