Looking through both the ASTM specification F3125 Grade A325 and the Research Council on Structural Connections (RCSC) website, we have found neither mentioning the use of lock washers in a structural steel connection. We interpret this as neither allowing nor forbidding the use and that it would ultimately be up to the engineer of record to decide. Tensioning the bolt properly should keep the nut from backing off, but in unusual situations with high vibrations, one may look to additional measures such as a lock washer or additional lock nut or jam nut.