The five-axis machining of a free-form surface often contains the reversal of a rotary axis’ rotation direction with linear axis synchronized with it. This paper proposes a new machining test to quantitatively evaluate the influence of the reversal of rotation direction on the surface geometry and roughness. In the five-axis machining, the trajectory of tool position and orientation is first given in the workpiece coordinate system (WCS) by the computer-aided manufacturing (cam) software, and the computerized numerical control (CNC) system converts it to the machine coordinate system (MCS) to calculate command trajectories. This paper clarifies that the tool path smoothing in the MCS can potentially cause a large contour error because of the dynamic synchronization error of rotary and linear axes. Although some academic works in the literature presented the smoothing in the WCS, many commercial CNC systems still employ the smoothing in the MCS, partly because machine tool users or makers do not clearly see how significant this influence can be on the machining accuracy. The proposed machining test enables a user to quantitatively evaluate it. The machining experiment shows that the geometric error of the finished test piece was as large as 0.16 mm under the conventional smoothing in a commercial CNC system, which can be significantly larger than the influence of other typical geometric errors of a five-axis machine tool. This paper shows, by numerical simulation, that the smoothing in the WCS can completely eliminate this contour error.