[Trevor Booth]

That set the cat amongst the pigeons!! (excuse the pun).
My experience -- commercial grade spring washers can and will break with high frequency vibrations. Mostly I have had this occur on specialist high speed machinery and have had it occur on some parts of racing cars and in particular the engine,trans area.
Simple brackets for alternators etc suffer HF vibrations. Another cause of failure is a stress reversal in the spring washer, say you have a bracket bolted to an engine block and the bracket , due to its loading, flexes a minute amount, the spring washer is alternately being "squeezed and relaxed" , it will ultimately fail. A correctly sized and torqued bolt will not come undone, the "holding tension" being provided by the proofing (stretching) of the bolt.
This is not always achievable on vehicles save for places like head bolts and like solid components. Most chassis related bolts need some form of security be it spring washers, belleville washers, conelock nuts, deformed nuts, nyloc nuts, loctite or lock wired. The choice is yours. My policy is to use hardened washers in all applications and particularly under belleville washers. If you dont use a hardened washer then the belleville (or spring washer) will "dig" into the soft washer and lose some of its installed tension.
The choice of bolt material also plays a part in the "staying tight" role. In lots of places I use bright machined MS bolts. I have seen many a car with million dollar bolts made from unobtanium come off a race track as a veritable "bucket of bolts"
Cheers