I do not have strong opinions on this yet as I simply have not played enough games, but I get the feeling that the current effective strategies [at high level, e.g. expert, ridiculous and beyond] are geared more towards peaceful rather than aggressive expansion.  

In the early game, where you only have a couple cities to defend, you can only afford to start a war that you can end almost immediately.  Without a clean kill I feel that both warring teams will be at a massive disadvantage later on.  Moreover, if you win the advantage gain is not as high as you would think, especially with captured city unrest level being the way it is.  I find it almost more advantageous in the long run to wipe out a city and put your own one in its place.

At the mid-game, it becomes incredibly hard to defend your empire with the exception of creating a choke point if you happen to start in a corner, which makes harassment much too easy for your opponents.  All in all, it's just not worth starting a war where all your outposts and shrines that you have taken years to build up are destroyed in a few turns.

It is only at the end-game, assuming you are going for a victory win, you almost have to have quite a number of stacks of doom to wipe out an empire quickly enough so as to not cause yours to become crippled.  The use of strategic spells can help defend your cities more easily, both offensive like pillar of flame and inferno and defensive like create mountain.

In conclusion, the opportunity cost for going to war is extremely high until the end game.  I think it would be more interesting to reward early aggression more than it is currently, but maybe I am in the minority.  

Again, I reiterate that I haven't played enough to come to any strong conclusions but I am definitely interested in what other people's opinions on this are.