Under my point of view (and inside my teaching philosophy), the scoreboard is a core concept of WBT that should be applied from the very beginning.
It's not just a tool for controlling misbehavior, but it's a tool that enables you to control and boost the motivation of your learners, their energy in the activities and, most of all, to show your learners that you are the leader of the group and that you are serious and constant with what you are doing. It's key to show all that from the very beginning, and that's why I'd use it from the first day of class.
And, of course, don't forget the scoreboard is fixed! And don't forget the +3/-3 rule!
Best,
Edward