Or, how about we just eliminate diplo hits for turning down a request to DoW? In SO many cases, its completely unreasonable that you should be expected to join a war. Just because Shaka really doesn't like someone, doesn't mean you shouldn't like them either. Now, if Shaka reeeally needed help, and offered up a tech or gold for you to join the war and you refuse, then maybe I'd understand. But just asking you to sacrifice troops/trade routes/traded goods and who knows how many "you declared war on our friend!" diplo points from other civs as a favor? Thats BS.
Alternatively, I think the player should get to list positive/negative relationship points with rival civs such that, if you are friends with a neighbor for instance, some jerk across the world doesn't even have the option to ask you to DoW. Make it a conditional list type of thing in the advisors menu -- if you share a religion, give the option to add positive points to your relationship status. If a civ DoWs you, make it an option to add negative points. Essentially you create a friends list and a s#itlist, so to speak, that AI must weigh in before it can ask you to stop trades, DoW, etc. Also, if a civ knows its on your s#itlist, if reacts to you different than if it was on your friends list. Maybe it piles defenders in border cities if its a strong civ, or maybe if its weak it'll cave to demands more readily. Sure could add an interesting and diverse facet to the game...