I thought we could collect our knowledge about Teppo units in this thread.
Recently, CBR held a little lecture on how to use them correctly which was very revealing.
The teppo units are used regularly in Multiplayer, 4-5 units are standard and form the skirmish line.
One can go without and replace them with Samurai Archers but it can be considered costly to have your
SamArchers shot by 100 or 200 Koku cheaper Teppos.
What was news to me and others is the fact that Teppos can be used to fire one volley from all soldiers.
The normal use is to deploy them in three ranks to get (almost) constant revolving fire. When deployed in three ranks, only the first rank fires. That's one third of the fire power.
If "Fire at will" is switched off and the unit is deployed in two ranks, you can manually fire a powerful two-rank volley. While 1/3 of the unit's firepower is suffice to reduce the morale of units crossing the nomansland, the full volley can rout units instantly due to the high number of kills and the morale penalty from being hit by a gunpowder unit.