I would just go ahead and run 12 gauge CL2/CL3 wire while you have the access. Most of the HTiB use 18 gauge wire but that can vary depending on the price of the system. I've seen some that actually use 12 gauge zip cord. As long as the end of the wire will fit in the clip terminal you will be fine. A lot of HTiBs, mainly Sony and Panasonic, have their own proprietary cables with plastic ends on them that plug into matching terminals on the receiver. I'd take a close look at the back of any system you plan on getting and make sure you can terminate at least bare wire if not banana plugs. Look for a description/feature like " 5-way binding post " in the literature.
Most of the Onkyo HTiB have these 5-way binding posts, so any shielded wire you buy ( like CL2/CL3 ) will fit in the hole at the base of the post OR you can terminate with banana plugs for a cleaner look.
Edited by jevans64 - 12/11/12 at 5:35pm