It depends on the terrain. If the enemy is coming to me anyway, I will usually just wait for him so that my missiles get the advantage of the first volley. But if he's not, it can be quite fun to maneuveur the enemy army from the top of a hill to the bottom of a valley without firing a shot (although rather cheap).

I probably should be a bit more aggressive considering that I fight a lot of night battles, wherein I believe the defender starts with a morale malus which gradually returns to normal throughout the battle, thus supposedly making a headlong charge likely to rout the enemy if done immediately.