If a pilot or other member of the coughpit crew wanted to deliberately crash their plane, one would hope the other members of the coughpit crew would (at least try to) intervene. Do you remember the SQ/Silkair crash in '97 or '98? The rogue pilot had to dispose of his co-pilot with an axe before he could set about flying his plane into the sea.
Since many coughpits contain three crew, I don't agree that it would be that easy for one of them to just fly the plane into the ground.
They could, however, easily blow up the coughpit with a small bomb in their shoe (or anywhere, really). Once the coughpit is gone, or even damaged, thats it for the whole plane...
So I say check their shoes, you never know....