If you're talking about split-ring lock washers, I would think again. They do not accomplish any kind of locking, and may make vibration-induced loosening _worse_ by providing a small opening force. They are useless for almost all purposes.
If you mean something like a Nord-Lock washer, then carry on.
but, not the service of enlarging the contact surface of the bolt or screw head.
Actually, they only perform a tiny fraction of the crucial services performed by washers and can be replaced by loctite in almost all situations, so, I'd contend they are the most worthless of washers.
But you don't always need more surface area, just something you don't want to wiggle loose, like say your cargo carrier on top of your car that's attached with hand wingnuts. I don't have enough experience with loctite to know if I want to spray it on that.
Yeah, a split ring washer there is probably not doing much. Possibly even making things worse.
Given it's more or less impossible to sufficiently torque a wing nut, there are actually not many great solutions...
A dab of low strength loctite would definitely help, but that's one-time-only and you've got to carry the loctite around if you want to re-do the connection... At which point why not just carry a spanner instead and just use a properly-torqued normal nut?
Probably the best would be to swap out the plain wingnuts for nylock wing nuts, which should at least help a bit.
I bought some split ring washers and they worked great. The nuts went from coming loose every few days on a long road trip, to not coming loose at all.
Lock washers help prevent fasteners from loosening particularly under vibration or similar stress. They're not as effective as LocTite or similar products (adhesive thread treatments), but also don't gum up the pieces.
Somewhat ironically, greasing threads can actually result in a more reliable fastening as it permits removal of the bolt or screw and permits stronger tightening which can increase the physical bond.
If you really want a reliable fastening, use a retaining device such as a cotter pin or retainer ring, or riveted fastenings ;-)
