Christian Dior, Haute Couture, AW14, Paris.
Here are a couple of examples of how princess panel lines were used in the Christian Dior haute couture collection for Autumn-Winter 2014. The example images show how the seam line can begin from the armhole or from the shoulder line, continuing down through or close to the bust point, and down through the waist.
Princess seam lines are a way of shaping bodice patterns to take into account the shaping around the bust and waist without the need for darts. In order for the seam line to do this effectively it has to pass over the bust point, or pass very close to the bust point. The further away from the bust point that you place your panel line, the harder it will be to shape the pattern pieces effectively without the need for additional darts or seam lines.
You can see a tutorial on how to turn a darted bodice pattern into a bodice pattern with princess seam lines in the post Fundamentals of Pattern Making: Princess Panel Lines»
Note how in the examples below from the Dior show, the white dress uses princess panel lines in combination with additional short panel lines. In this situation you would toile and perfect the fit of the basic princess panel shape first, before you divide the pattern up into additional pieces.
Images from Vogue.co.uk»