To a point you are correct, it is not required to use the exact route given...however that route does include specific fuel stops. If those fuels stops aren't needed, or there isn't fuel set into the route, you can pretty much go whichever way you feel comfortable with. There is one other catch, Swift pays their Highway Use Tax, based on the computer routing, so if you go outside of the computers route, you (technically) should contact your DM about the route you are planning to take, or have taken.
