Mitered Joints conceal the endgrain of the wood. The endgrain is the roughest part of the wood, and also the most vulnerable to wicking up water. I use glue at this connection as well, making it waterproof. Cedar tends to stay together and stay level better than green treated lumber, but I use the same technique on both.

It is not always best to miter connections--for example, on cedar fascia, a mitered corner tends to separate. It is better to plan the seams in fascia to be under the mount area of the railposts.