Wow, Ironside. Talk about having your assassination strategy backfire on you.... Unless I'm truly desperate to eliminate a rival, I try very hard to make sure I'm allied to a clan before killing off its leaders. This way there's a decent chance I get at least some of their lands.

That said, I don't use ninjas very often (and geisha almost never, as they're way overpowered), except as defensive agents to capture enemy assassins. I generally prefer to destroy my enemies the old-fashioned way.