Should you include tone of voice in the guidelines?
Yes, if it fits on the same sheet: three lines you would say while setting down the coffee and three you would not. A chapter on tone is unnecessary if the team is not going to open it.
Write to us. A quote with no commitment.

Note how you actually greet people at the first coffee and at the last drink. The paper says a warm manner and at the bar the reply is rushed, and the review notices. Those six lines beside the mark, on the page the shift can read.