Here is what Boeing says:
Fuel Jettison
Fuel jettison should be considered when situations dictate landing at high gross weights and adequate time is available to perform the jettison. When fuel jettison is to be accomplished, consider the following:
• ensure adequate weather minimums exist at airport of intended landing • fuel jettison above 4,000feet AGL ensures complete fuel evaporation • downwind drift of fuel may exceed one NM per1,000feet of drop • avoid jettisoning fuel in a holding pattern with other airplanes below.