So-called waterproof down isn’t actually waterproof, but water-resistant. If you dunk it in water, it will fail to insulate, although it will dry faster than untreated down. Therefore, if you plan on sleeping in snow caves or out in the open in a bivy sack, waterproof down is probably a better option since the down will dry faster if the fabric shell of your bag gets wet. If you’re sleeping in a cozy tent with a waterproof floor, it probably doesn’t matter that much, except if you’re on a multi-day trip where the accumulation of perspiration passing through your insulation and out the breathable shell of your sleeping bag can accumulate in the down, degrading its loft and ability to retain warmth. Waterproof down would be better in those circumstances.