For archers, yes, I'd say: use the arrows only when they will do much damage. For teppo... ehh, they have almost unlimited ammo anyway?
Most of the time I'd agree with you. But..............in my last Oda 1580 camp I had 3 teppo with my defending army at Mikawa and all three were run out of ammo with FaW disabled

I think there's a screenee of that battle in my Nobunaga's Ambition AAR..............

Yet, I have no clue if this little strategy of ours really pays off in numbers. I guess so.
If the number of dead bodies after a bridge battle is any indication, then I would think it does because the greatest concentration seems to be always on the bridge itself....................