There are many devices that can be powered through USB, even reading lights etc., so if you go for 5V DC in the house, there are already many devices on the market that could take advantage of that. I also have an adapter that gets me USB power from either a car's cigarette lighter or a wall socket, so this solution is backwards compatible, i.e. you can use your new 5V-wall-powered devices even where no 5V wall power is available.