I believe that knowing how to replace a spoke and true a wheel (roughly) while on a tour is a good skill to have. However, knowing how to build a wheel, while a good skill to have, is not (edit)
necessary for touring.
I think of the wheels as the most important parts of the bike. You can jury-rig a lot of things, but an inoperable wheel is a show stopper. I prefer to let someone who has built 100s, maybe even 1000s of wheel sets, build my touring wheels. At Universal Cycles in Portland, I can get custom built wheels for just about the price of the components. Their wheel builders are excellent, and proven. Between my wife and I, we have put about 50,000 miles on 3 sets of their wheels, and have only had to true one wheel once. FWIW: Velocity Dyad 36 spoke rims, Wheelsmith DB spokes, and Ultegra, 105 and XT hubs.
I hit a large piece of metal debris while descending a hill into Medicine Hat, Alberta. I was going about 30 mph when I hit it. The force was enough to blow the tire and dent the rim. Even with the damage, the rim was still true and round! Luckily, Medicine Hat had a bike shop that had one wheel that would work.
I called the guy who built my wheels, asking him about bending the rim's sidewall back into place. He did not recommend trying that approach. If I had disc brakes, I would have continued on, but we still had about 1000 miles to go and I wanted my front brake.
You can see the bulge on the left side of the rim. It extends inboard, and can be seen on the right side of the machined braking surface to about halfway to the spoke hole. I'm not sure a wheel built by a less experienced builder (me) would have fared as well. I almost cried as the mechanic cut out my hub so I could mail it home.
