For the devices that run off of low voltage DC today, lots of other commenters make good points about needing to regulate the voltage anyway, and I'm inclined to agree. Those devices generally use very little power anyway, so it probably isn't worth running a second set of wires because copper is expensive.

On the other hand, the biggest power users in a house (heater, air conditioner, dryer, stove, refrigerator) seem like they are a different story. In a future home with solar power and a high voltage DC battery pack, I think there is opportunity to make appliances that operate from the raw battery voltage, which is in the 300-500v DC range. That way, there isn't any conversion required at all, and those applications should be able to handle slight variations in voltage as the battery discharges.