Whatever wire guage you use, as long as you have any say in it, don't let them staple the wire to the 2x4's inside the walls - the usual consrtuction practice and not a very sound-friendly thing to do. Work out some other solution that neither uses ferrous metal fasteners nor pinches the wiring when secured to the framing. The physical pinching can lead to a somewhat narrower audio bandwidth - easily avoided when having your home built, but to be sure you will probably have to be physically present to keep the workers from doing things the way they're used to.
