I'm assuming this is a bolt-on neck.

I think this is really in urban myth territory. I've known people who've had guitars with shimmed necks where that were done over 40 years ago. I've shimmed a lot of necks in my time and I've never had a problem.

The pocket end of the neck (including the fretboard) is normally the thickest part of the neck and its also the squarest. So it's definitely the strongest part of the neck. So why would that warp and not the thinner contoured part of the neck that's probably got 2/3 the material of the heel in cross-section. Not forgetting the nut end has the most leverage force applied through string tension?

I'm not one to say that it could never happen, but you'd probably need a whole combination of factors such as repeated big temperature swings and large humidity changes, spread over a long period of time before it's ever likely to occur. If it does, you're probably also looking at wood that has inherent weaknesses in it (as not all necks are made from the ideal combination of quarter-sawn, very straight grained wood). In many instances you'd probably find those frets would have become loose anyway, but because the neck has been shimmed, people then think that's the primary reason. The fact that the fret slots are closer together at the treble end may possibly have something to do with it. But if it happens, it's rare.

I personally wouldn't use metal washers as shims. I feel metal is too hard. The luthier books normally recommend wood veneer or hard plastic. I normally use a strip of thin hardwood veneer, about 10mm wide, across the full width of the pocket. Thin veneer is somewhere between 0.6mm-0.8mm thick. If I need something a bit thicker I'll stick two strips together.