You should always have divisions organised under some commander. I haven’t had time to play since the reorg of field marshals versus generals, so maybe there is an advantage somehow to just having divisions under an FM (though I would think FM + general should always be better). Never leave divisions without leadership thought. Having a chain of command always gives you bonuses. Field-training is also a great return for your investment. Veterancy is a huge combat advantage. There is no point in training once they are regulars though: it just burns your equipment.