I sometimes disagree with the opinion that geishas are unbalanced. In terms of game mechanics, the ability to use and be abused by geishas is basically what keeps the game from continuing in to the year 2001 (or beyond) by pokey players. The feature adds an artificial timeline that whereby you should have things wrapped up in a moderate amount of time (50 years is average) so it rewards the player who is more decisive. In the words of the Hagekure, "A samurai is one who makes a decision within seven breaths. It is a matter of thinking things through and then being decisive that lets one break through to the other side." So I suspect that sitting back and just making armies that you never intend to use is very un-Samurai-like.
Anyway, I usually finish the game before the enemy has the resources or time to get geishas.