Considering your earlier posts and your assumed focus I take it that your asking in reference to 66 San Jose. In that case yes I've always seen a washer used on the bolt that enters/ attaches to the bottom of the rear frame rail
Yes, but what kind. My two San Jose Oct 65 builds had a split trapezoid lock washers (34783-S2, 7/16LKW, SPT, 0.446-0.464ID, 0.784OD, 0.121/0.091T, TPZ).
As documentation for the tie down brackets is not available, and this was an area that quality was primarily interested in "does it work", and not "does it look good", almost anything can be found.
Jim