Even in a large residence I can only see there being a handful of appliances that require "240V" but most of them like electric water heaters and motors rated "200-230V" (as are most central A/C units I have seen) will operate on 208V with no problems.
The only place I have actually seen a buck-boost transformer used on "residental" appliances would be when I was in school, where the cooking classes had them on the electric stoves so they would heat up within time, and in the office where they had a window shaker A/C rated at 230V and would not start at 208V. Electric water heaters and the like were connected right to the 208V supply.
