This post shows in detail how Herb braces his wooden fence posts and gate posts.
If you aren't sure the leveling is important, do this experiment. Take a box, fold its top and bottom flaps in, lay it on one side, and press down on it. It will not collapse easily. Now take another box, fold its top and bottom flaps in, push the top sideways to make the box into a trapezoidal shape, lay it on its side, and press down on it. It will collapse easily. I bet you never knew you needed geometry to build a fence! :)
I figured Herb knew what he was doing, but I was quite surprised to see what tightening that one wire did. The photo on the left shows pre-tightening. The photo on the right show post-tightening, and you can see that the gap between fence post and ground has widened. That's because simply tightening that wire pulled the outside fence post closer to the middle one.
These two photos show even more clearly what happens. The gap between cross-brace and fence post on the left has been completely closed in the photo on the right, after the wire has been tightened.
Lord, bless the hands that have done this work.