This is a little guide that explain how(and when) Yojimbo will use Zanmato.
It depends by many factors,but one the most important is the affinity.
When you get Yojimbo,this parameter will start at 50 points. The minimum is 0,the maximum 255. The parameter:
-decrease of 20 points if you pay it 0 guil.
-decrease of 10 points if Yojimbo died during the battle.
-decrease of 3 points if you make him return after you summoned it.
-decrease of 1 point if he uses Daigorou
-doesn't change when he uses Kozuka
-increase of 1 point when he uses Wakizashi on one enemy
-increase of 3 points when he uses Wakizashi on all enemies
-increase of 4 points when he uses Zanmato.
More hight is the affinity,the more times Yojimbo will use Zanmato. Unfortunately,you can't see this parameter in the game,but if Yojimbo uses Wakizashi (or,more better,when he uses Zanmato!) without being paid,that means that the affinity has reached the maximum.

Excuse me if the English is not perfect...

-Thanks to:
Zell's Fantasy
and
gamefaqs.com