I'd put it in a spot where you're not finding yourself sliding forwards or backwards too much. And as I noted, putting the saddle further back usually requires something of a drop in the saddle height.
Keep in mind that my advice is simply gained from my own riding. I have no other knowledge and have never been trained, other than that I always wanted to emulate guys like Greg Lemond or Bernard Hinault or Laurent Fignon. All of whom were Guimard pupils. And it seems to work for my body for the most part.
http://fastrunrobbie.files.wordpress..._11a_widec.jpg
http://www.cyclesportmag.com/wp-cont...e-france-3.png
Contrary to popular belief, this is not a painful way to sit on a bike. On the contrary, it is comfortable and the bike handles properly.