I guess it varies from Company to Company and Country to Country. In India, DGCA clearly says that for seasonal wind one must carry 10% contigency fuel whereas for actual wind we are allowed to carry 6% contigency fuel.
Ruddman,
If you look at an individual case 300-400 kgs doesn't sound much. But if you have lets say 100 departures a day, that 300 kgs of fuel equates to 30 T of fuel, that is a lot of money. Most of the airlines have more than 100 departures a day. As Intruder said many companies track the fuel usage.