IMO, they should be at the front far enough away from the edge of the tee pad that a player throwing from the very edge would not hit the sign with their follow-through.
Also, I prefer signs that are placed perpendicular to the fairway, i.e. when you're reading/facing the sign you're also facing the fairway. I've seen a decent number of courses that mount the signs parallel to the fairway so when you read the sign you're looking perpendicular to the fairway. For courses without defined tee pads that can be confusing.