I personally think as flat as possible is the best way to do it. I don't have any numbers or theories to back that up, but it just seems logical that the closer you can have the two surfaces, the better.
Also, you guys see this article at Spode's Abode about Direct Die Cooling?
http://www.spodesabode.com/content/article/directdie1
Also here he compares it to a Maze.
http://www.spodesabode.com/content/article/directdie2