The only ways I know of to influence troop placement are weapon choice and (possibly) champion class. If you give your champions bows or magic staves, they will deploy towards the back of the party, while if they have swords or similar, they will deploy on or near the front line. Be advised that champions with ranged weapons will deploy much, much further back than your close-quarters fighters will, and this can be an issue if you want to get that champion up to the front in time to cast a touch-range spell, such as Tame, on an enemy.
I suspect, though I am not certain, that the class you choose for a champion also influences their placement location - Defenders and Warriors towards the front, Mages and Commanders towards the back. Remember, though, that unless you give your champions ranged weapons, "towards the back" probably only means a tile or so further away from the enemy than the rest of your troops - and if all of your troops are archers while your wizard carries a dagger, your wizard will be your front line.
