[Gandalf Parker]

And like many recommendations in the game, map size can matter (or actually prov-to-player ratio). If you have areas that you own which would take you a few turns to get an army to, then local defense becomes more important.

For Dom3 all answers can be considered to be followed automatically with "which of course is only usually true depending on your nation, solo or mp play, the size of the map, how late in the game, which mods are in use, game settings, and who you are presently worried about".