I always tend to take a "whatever is available and seems like it would work given the expected opposition" approach. Which in most cases shouldn't really be very unhistorical - commanders tended to have to make do with what could be scrounged up, and the smart ones at least tried to take steps to counter any specific competences the enemy might possess (eg. lots of archers mounted and foot if you're going to chase steppe-nomads, lots of spearmen if the enemy is known to favour heavy horse etc.).

Although granted, when possible I do tend to build them armies to certain basic and usually more-or-less symmetric patterns.