That's a good question. I perused the REI website and of 94 down bags, 92 are supposedly water-resistant. Then I filtered the sleeping bags for really cold by selecting -50-0 (F) for temperature and then looked at the results. We have 14 bags out of the filter, nine of which are down; of those nine, six are "water-resistant" down (looking at the filter count on in LH column).
*Note: when looking at the bags closely, the Nemo and all three Marmot bags specify down and do not mention "water-resistant", so there's a small discrepancy in the count, but water-resistant down is definitely in the majority (if that means anything).

I would probably go with water-resistant simply because the prospect of a critical item failing because it got wet is unacceptable. Even with great care, stuff happens. That said, the Marmot bags use a waterproof breathable shell, and Nemo uses a coated nylon shell.

... and with all that said, this thread is making me look much more critically in my choice of bags, mine are all synthetic, three season bags.