Not sure how you reason, those flat washer do nothing to prevent the bolts to unscrew!
Biggsy's right, use Loctite threadlock - it is the only thing that last. Similar compound is used by the industry for example on the bolts that attach the brakes to the frame, as the manufacturer might get sued if these go missing!
edit:
Another better way is to tread a extra long bolt from the inside of the dropouts and put a nylock-nut on the outside! That way you never lose the bolt and if it break you can easily remove what remain. I would still use thread locking compound as it also prevent the threads to oxidise/rust.
Also use big washers to spread the load to a larger surface to prevent cracks on the rack. That's what they are for...!
Just make sure the head of the bolt clears the chain on the drive side.