Just remember that its not 'if' the builder will screw you, its 'when'. Its the nature of their business. They arent building houses for fun, its for money and lots of it. Usually a fixed percentage from what I have been told. Somewhere in the neighborhood of 15% - 20%. If you give this prewire away, will the builder pass on the savings to the customer? Do you have any way of enforcing that if he agrees to it? Will you warranty your work? If the sheetrockers shoot a screw through a wire, will you have to eat the cost of replacementof the wire? Or, if the homeowner decided to hook up his exisitng gear to your wiring and there was a short that damaged his gear, will you have to warranty that?
Just a few questions that came to mind when reading this thread. Good luck at making it work. Let us know if you decide to do this and how it works out for you.