Its actually good advice for ALL the nations you take on. Ususally what I do is corner the remnants of a defeated foe in some non-vital, non-strategic territory and let them sit there until the end of the game (when I generally take all these little islands of resistance out in a single turn), or until they finally lose all their heirs and go rebel. It helps to prevent them from not coming back in. With the Pope though its a must…though I usually take Rome and push him back to the eastern territory on the Italian peninsula (which I can’t remember what its called on the map now either), and let him sit there throughout the game. BTW, its good to keep a high level assassin near the pope…its a great way to get out of being excommunicated. 
-XT