Yes, you can, but there are a couple of things to keep in mind. One is the thickness of your posts. On the sides of your posts, the depth of cut is at its maximum. If your tool is not long enough, you can hit the workpiece with the collet on your spindle or router. The other is that the design on the sides of the post will be distorted by the fact that the cutter has to come straight down from the top and can't really cut in from the sides (except for very simple shapes that can be cut by a side-cutting bit). However, if your design only covers, say, a quarter of the post, then you can get reasonably nice results cutting it flat.