Flying visit, you have to remember that the flight planned fuel is the minimum legally required for that route. It does not take into account ANY delays that might happen. It also doesn't include any holding fuel so if I'm reasonably sure that we will be holding somewhere or there is adverse weather I'll take a bit more. How much extra really depends on the day. Our company policy is to carry 800kg extra if we are expecting a CAT II approach at our destination plus any extra the captain requires.
So the short answer is yes I can take as much as I want but I wont take more than I think is necessary or unjustifiable.
I should also point out that although the captain has the final say on the amount of fuel that is carried the first officer should be included in the decision making aswell.