A DC offset would have the negative side effect of causing AC wall transformers to saturate, redering them useless. Also, as far as I know it would cause problems for many kinds of AC motors. True, you could bypass the DC offset with a series capacitor, but then you would suffer efficiency losses in larger appliances, in addition to added cost and complexity. The problem with having +5V centrally regulated and then distributed throughout the house is wiring resistance losses. Let's say you decide to use 16AWG wire, and have a run of 100' to a hypothetical outlet (not uncommon in even a mid sized house). that means your round trip length is 200'. Given the resistivity of copper, the resistance of such a wire is 1.26Ω, and with .5A of current your voltage drop would be .63V. Therefore, 5V would leave the distribution box, and only 4.37V would be available at the outlet. Worse, Pin=5V.5A=2.5W, Pout=4.37V.5A=2.19W. Efficiency of just the transmission system is 87%. You can get a good power factor corrected switching converter that can do better than this just plugging into the standard wall outlet.