Good post Brian. That's the kind of thinking that leads to better understanding of deer behavior. I don't neccessarily agree or disagree with your observations regarding sap content, but your thoughts on rub lines and travelling bucks are outstanding.
When you think about it, rub lines lead to feeding areas and bedding areas. Those are the places to find does. The buck that made the rub line is spending time in the area so he's going to be taking advantage of the terrain and wind in his travels. Leaving scent on the rub assures other deer the sign is fresh and was actually made by a deer. What better way to ensure breeding success than to tell every buck who comes by where the does are and the best way to get to them.