It doesn't matter too much, provided (1) you aren't that armoured and (2) the battle isn't that big. If the enemy has a wave of reinforcements to bring on, and your armoured troops are already fatigued from fighting the first wave, you take WAY too many casualties.

I notice it most with cavalry, who I suppose do more running about. Even MS tire after chasing one wave of routers and then never really recover to be that much use. using knights in the desert seems to be a good way to get them killed as they tire so fast and the penalties are massive. Hobilars are ideal.