I play on emperor and buy embassies as soon as I need ROPs. I usually sign a ROP with any civ that I am not interested in or unable to destroy, almost always for free or cheap. This will boost your rep with surrounding civs and will prevent the huge tidal wave of other civs coming to the aid of my victims(where my victim gets a civ to ally against me, who then gets a civ to attack, who then brings a friend, on and on). This seems to be very common on Monarch and up levels. So all the people who cry about the AI always ganging up on them read this and do it. If the AI civs do try to pile on, it will cost them more because of your good rep with others and will prevent most if not all intervention. In conclusion, I buy embassies with all around me asap, mainly for protection through ROPs, then start picking off victims. When ROPs come to an end(20 turns is shorter than you think early on), find new victim and sign new ROPs with the others.