Actually, I use them extensively at times, especially on the HRE, which seem particularly vulnarable to them.

As to provinces, I keep a bishop/cardinal, assassin, and spy in each of mine. However, I have a bad habit of demanding perfection and not building spies or assassins until I can pump them out with max stars...and in Syria if I have it.

Which brings up a question. On my map, it doesn't say anything about bonuses to assassins in Syria, nor Katanks in Big C. Are there any other provinces that give a second valour bonus beside the one mentioned on the map?