The "courses" (airways) over the Pacific are 4 miles wide, and separated by 30 miles. It is usually only over open water, where there are no VOR/DMEs for updating, that errors more than a half mile are generated. Once in range of a VOR, DME, or LOC, the errors in the FMS are reduced again.
We stay so close to course that it's sometimes scary... Over a North Atlantic Track during "rush hour," a pilot may see 4 or 5 airplanes, all lined up perfectly in front of him, and several more on TCAS within 20 miles behind. Previously, before ring laser gyros and GPS, errors of a couple miles laterally were common, so the "big sky, little airplane" theory was credible. Now, the nav systems keep us so close to the magenta line that a controller error in altitude assignment, without an accompanying TCAS alert, is much more likely to have dire consequences.