When the head develops that ridge, it's only usable if the groove is aligned perfectly with the tape path. One scenario that can and does happen is you have a groove in the head that wasn't adjusted correctly to begin with. Case in which you can't adjust it unless the head is re-lapped. Because what happens is regardless of your adjustment, the tape will tend to fall back into the groove, which will mess up azimuth. In regard to the gauge, wouldn't recommend jumping on one unless you need to perform such checks/adjustments in the future. Good quality ones are very expensive and not worth unless you use them on a regular basis, while many cheap ones don't have the needed flatness tolerances.
