I believe Contingency fuel ( 5% of trip fuel) is only to be used for IN FLIGHT re-planning purposes in case for example your initial alternate becomes unavailable..
My 3 cents...
But I believe we all agree that if missing 50-100kgs of fuel before takeoff , one may elect to go rather than delaying the flight.(knowing that enough fuel is available for the destination and possible alternate).
However a more detailed planning of fuel uplift will cover expected delays during taxi and avoid too many head scratching on the holding point..
My 4 cents
