It is simple mechanical engineering, Grunt - you need all of the thread to be in contact to cover shear stresses in the threads. It is usual good practice to allow three threads to show through. If it is prone to damage or corrosion, other preventative measures should be used. If all the thread is in contact then the corrosion can be removed when the nut is undone. a change of nut type from, say, nylock to castellated will overcome some of the problems.