Not to take anything away from Chuck's very good advice, but I would resist placing signs beyond the front edge of the tee pad as they may be considered a sight distraction. Possibly even a stance obstruction if your tee shot is really bad! Ultimately though, that point is a matter of personal taste. The main objectives for me in this task have been to place them so they can be seen from the path from the previous hole and as close as possible to the tee pad without being in the way of players who like to approach from the severe angles Chuck mentions, as well as the more traditional approach from directly behind the pad. That way, players can review the sign on the way to the pad. Realize though, that may not be possible or desireable on every hole.
I also feel that installing them at a height somewhere between the hip and knee results in a much more elagant look that at face height. Signs placed at this height will need to be angled for ease of review. For me, signs that are at face height and 90 degrees to the ground are not very inviting as they obstruct the panorama of what should be a beautiful setting. I know that probably sounds esoteric, but in many cases, it's the small details that make the difference between a good course and a great one. For an example of this, check out the course reviews for New Quarter Park in Williamsburg/York County Virginia (a course that Chuck helped me with!).