Hi Matt,
As a teacher, I really welcome messages from the student before a lesson. It helps me prepare.
If the student books a lesson but doesn't send a message, I usually write a friendly message asking what they would like to do in the lesson (I mean, on the booking page, right had side). If there's no reply, then I'll have a few things planned and take the lesson as it comes.
From a teacher's perspective, I suppose all we need to know is what you want to do in the lessons, or what you want to aim for when practising the language. Even if it's just chat, it helps a lot for us to know.
Have fun!