I built over a 1000' of welded pipe horse fence around part of my pasture. I made the top rail at 52" to 54" off the ground. Anything taller looked aesthetically displeasing. However, for a little extra height, I've seen people put a hot wire along the top using the insulators that have the bracket for chainlink fence (From TSC) If you attach it to the top rail (in my case tack weld it) and point it straight up it gives you about an additional 6" of height. The electric wire helps prevent the horses from using the top rail to scratch and pushing the fence out of whack.

Regarding following the contors of the land: I set all of the poles and then run 100' of string where the top rail will be. You don't want to follow every dip and hump in the land, the top rail would look ugly. I just keep stepping back and adjusting the string height to give the best look, while approximating the 52" to 54" desired height. Some points along the top rail may be greater and some less then the 52" to 54". I mark the height on all of the poles and then move the string to the next 100 foot stretch and repeat.