Zouave:
Planting a spy to start a war is an expensive way to go.
If you are worried about your rep, I can see why you would try this. I only worry about my rep when my army is weaker than my opponets (this is true usually up until I fight my first war) after I fight my first war, get my army built up, have my first GL & army, I could care less about my rep. Also: if you have a comparitive advantage over another civ, they are less likely to declare war on you, then you are the one who has to break the peace.
The best use of planting a spy is to see the actual strength of a civ's armed forces. Using this function you can choose with a good degree of accuracy the civ's you need to conquer (very useful if you have more than 1 civ on your borders).