I haven't used agents to make rebellions yet, or let's say I tried but didn't see any results. I was sort of trying to do it like in Shogun, where you make rebellions and then go in with your army to take regions without officially declaring a war but so far, that has not worked for me yet. I did go and conquer a few towns where I tried to make a rebellion before and suddenly had them rebel while I was there, which was a little amusing. It made my conquest of Italy a little more exciting, since the rebels put up more of a fight than the Romans >_>