It's sort of like that. Before a battle, units are set up how the computer would if it controlled them.
Depending on who I am fighting, I set it up in different manners.
1)If my cavalry is superior, I balance it out on the flanks, while the infantry is arranged to engage the enemy infantry.
2)If my cavalry is equal or inferior, I make a massive cluster of them on one of the flanks, and decimate the enemy cavalry on that flank, then swing around and slaughter the remaining cavalry. My infantryis set up as already said.
However, if fighting the Germans, Macedonians, Greeks, or any other Factions that start with some sort of spearman unit, I arrange my infantry so they are a good distance apart from one another to prevent multiple units from being flanked and trapped. This also aids flanking the spearmen units, as they all branch out to engage a different unit.