Dave, I have done it both ways, cut them all the way across and just the six inches. If it is just pure function, it can be cut all the way across. I now prefer to make the tenon flush to the non reference side, 4" and 6", the mortice is cut and no need to clean on the other side of the mortice. Only one shoulder to cut as well, using the french snap this would be a very simple and quick joint, allowing one to use hand tools in a modern world and still be right there with the big boys and their big saws.

Tim