The hardware and instructions supplied by big name racks like axiom, bontrager, do not indicate a washer to be sandwiched between the bike and rack.

Lock washers are usually not supplied or required.

But you can do whatever you want, there is no harm in adding extra things to the fastener, including blue loctite.

I install a lot of used racks onto bikes, and the only new fastenrs I provide are the M5 or M6 hex bolt with one washer.