Early game i am always using my spymaster's 2nd option (the one that grants +% of assasination sucess/-% of assasination discovery), because it occasionally provides you with +50 gold, which at the start is pretty much needed specially as a count.
Once i am big, i am always using my spymaster to prevent vassals from entering "factions" against me.
I only "spied" when i started playing, and i always spied venice.
Either that, or when i want my wife to get killed and i dont got the money for the deed - putting your wife as a spymaster and sending her to someone with a great spymaster is a great way of getting a new chick.
