According to a NASA fastener design manual[1](page 9) split-ring style lock washers are useless for locking purposes. They say that star washers do proved some additional friction/locking action, but at the cost of damage to the surface(the friction comes from digging into the surface).

[1] http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/1990000...

Thanks. That's great to know. And lots of other useful information.