The accuracy requirements on surface placement depend on what they will be used for. For example, for EEG/MEG source estimation, a surface misplacement of one or two mm will have little effect on the computed solution. Similarly, for analyzing and visualizing functional data that is typically acquired with voxels that are greater than 3 mm on each side (although this is changing!) surface accuracy is not critical. However, if one's goal is to measure morphometric changes associated with disease state, neuropsychological variables or age for example, one is typically aiming to detect changes on the order of ½ mm. This naturally places significantly greater constraints on the geometric accuracy of the resulting surfaces.
