Theoretically, the involute profile of the gears will give perfectly level motion. But of course, the gears are not perfect, and there will always be a tiny amount of up & down movement. In the prints we've tested, there is no noticeable up & down wobble in the print surface. (I'll try to get some hi res photos to post soon.) Also note that the main benefit of a really fine layer thickness is to hide the layer lines, and the layer lines still disappear even if there is a tiny about of variation in the build platform height.