It should logically follow that the point of an aerofoil which produces the most amount of lift (indeed the point at which the Centre of Pressure is located) is coincident with the point of greatest Camber. From my knowledge usually located somewhere in the first third of the aerofoil, behind the leading edge.
Putting a spoiler anywhere else would still provide a means of dumping lift and increasing drag, but it would be far more effevtive to put one (or many) at the point of greatest Camber.