I thought Senior Dispatcher gave a very good explaination of how things are done in the US. The dispatcher and the captain are jointly legally responsible for the safe operation of the flight. There is a little extra information to go with it though. The dispatcher often has detailed in for on winds that the captain doesn't have as well as a lot of computer processing power. At my airline at least either the dispatcher or the captain can add fuel unilaterally, but must get permission from the other to DECREASE fuel. The number the dispatcher enters is not just a default number, but is arrived at by some pretty complicated programs. I very seldom add fuel and the only time I have ever ran really low I thought I was starting out with a lot extra-a lot of things just went wrong on the same flight. Of course the captain always has the ultimate authority to do what he thinks best but he doesn't need to be a ass about it unless he has some serious self esteem issues like a few posters here seem to have.