How to build a fence on an unlevel surface?

I am planing on installed a large fence (150ft) and it slopes downhill a bit. Instead of using pre-fab 6x8 panels I am just going to make my own using 4x, 2x4 and standard dog ear slats. I am going to space the 4x4's 8' apart but then when I attach the 2x4 supports do I just go a certain distance from the ground and keep it the same or do I level the top of the fence or do I just put the dogear pieces the same distance from the ground making the top of the fence slope with the ground? I am trying to avoid the "stepped" look of a prefab panel.