I rode across the country with 12 other people. That amounted to more than 48,000 bike miles. Not one cable issue. I have never carried extra cables on any tour. But as noted, they weight next to nothing.

Where exactly will you be touring in those areas? I am scheduled to fly to Rapid City on 6/17 for about ten days in the Black Hills and NW Nebraska. (FYI, there is a bike shop in downtown Rapid City called Cranky Jeff's.) When I rode cross county we did ACA's Northern Tier route, which at the time used the entire Highline from Cut Bank to the ND border. ACA recently changed the route to avoid the oil related traffic around Williston and those parts. Shame. Rte. 1804 was a nice ride and virtually devoid of traffic.

I was just in Butte again last year during a loop from/to Missoula.