Overwing exit coverage is usually required on the airbus 320 and 319, at least one pax per row, however if the aircraft is not full and the spread of passengers is low particularily in the centre of the aircraft the SCCM can decide that it is not nessacary to move a passenger there. ie if it were obvious that passengers would all use the main exits.
Hope that helps and I am sure it varies between airline to airline and aircraft to aircraft though.