You have only one opportunity — make the most of it! You took the time to position the electrodes accurately, so it should be worth the time to measure their locations with respect to surface landmarks to enhance future signal analysis options.  3D locations of electrodes can be used in co-registration of surface measured physiological signals with anatomical MRI, CT and other imaging modalities in software applications such as EMSE Suite and others.  Quick and accurate measurement of 3D electrode positions adds only 3-5 minutes to a typical study, but it ensures precision when coregistering surface measurements with anatomical images.