In my opinion, you should not disturb the bed once the bed has been vorlaufed and runoff begun. Stirring the bed means that all those fines that were immobilized near the top of the grain bed during the vorlauf, are now mobile and in the wort. They would then be carried into the kettle where they can do damage to your beer.
If there is a concern that your bed is becoming 'stuck', I find that its often because of formation of a schmutzdeck (dirty layer) at the surface of the bed. Some brewers like to 'cut' their bed with a knife, but I believe that gentle mixing of that layer into the upper inch or two of the bed, will improve permeability better than cutting.