Well, once my army is large, I probably won't be building any forts, as I'd expect my borders to expand faster than forts could keep up. As, say France or England, why bother with forts when your army/navy should be crushing anyone that comes near you?
For smaller nations, the posts suggesting a fort every 4 provinces, a province away from the border, sounds optimal. But that assumes resources allow. If not, I'd want to cover my best provinces first, and try to place forts in mtns.
It'll be interesting to see how the changes work. Under the old system, I never fortified a province unless a mission requested it.