DL,

With center line layout, you would just snap the line longer than the timber, and plumb down from the ends of the timber. To follow up on my earlier question, in my mind, a king post would be centerlined.

With edge layout, an offset layout line is often snapped so that any wonkiness in the timber doesn't get in the way of alignment. Check out the current TF Magazine article that Tim mentions. I'd stick with the top and outside edge for posts, plates, tie beams, rafters, and joists. I have used the bottom of the tie on and English tying joint as the reference on a pergola, where the upper surface wasn't relevant.