All good replies...
I will only add(after doing 6 acres - various paddocks) make a story board pole. Take a piece of wood and mark your rail heights on it. Put it next to each post and mark a reference point on the post. Essentially your are marking the elevation of the terrain. You can than decide to either cut the post off before attaching the top rail or attach top rail and cut. The wood block gauge on the top rail will give an overall even appearance. You can slant the post top using a chainsaw or for a finer cut a recip saw with a good wood blade. One other approach, from an old timer - is to find an appropriate tin can(cat or dog food or soup) and cut it down to 2-3 inches depending on your space and cap each post. He has done this for years and all of the posts(we share a fence-line) are sound - 20+ years.