if jumping one car from another it is prefable to disconnect the negative lead first, mainly because there are earthing points all around the engine bay , even the bodywork, if you were to disconnect the live first and accidently drop it or make contact with an earth you would be shorting a battery out and big sparks would follow.
i was told that the ideal way to attach leads is to put the live on the battery, and the negative on an earth point away from the battery this is to stop any sparks occuring near the battery,