Fence posts should always be made of a weather-resistant wood, such as cedar, redwood, or pressure-treated pine. Just as important, the post tops should be cut at an angle of at least 30 degrees to shed water. Otherwise, water soaks into the porous end grain, swelling and splitting the wood and making it susceptible to bowing, twisting, and rot.
While it's easy enough to cut the angle by eye, measuring it lets you angle all the post tops identically. The best way: Set a sliding bevel square to the preferred angle with a protractor, hold the square against the post, and mark it. Then cut along the line with a handsaw; a reciprocating saw or chain saw is faster, but it's less accurate. Afterward, seal the angled cuts with two coats of clear wood preservative, stain, or paint.