Depends, for some reason not all units like being stacked. (they spend time trying to get room instead of actually fighting when the heat is on) There's also the missile troops to take into account. Missiletroops that don't get their own space will not fire, (they swirl their slings and draw their bows, but no missiles are actually fired.) And in the regard of your opponent's missile troops, a dense block of troops is an awfully tempting target, you're bound to hit something.

That said, there are times when it's useful. and some units like pikes(phalanxes) like stacking pretty well.