For want of a more natural proposal, it is generally assumed that the back-reaction of a quantized matter field on a classical metric is given by the expectation value of its energy-momentum tensor, evaluated in a specified state. This semiclassical theory can be reliable only when the fluctuations in the energy-momentum densities of the quantum field are negligible. Based on this condition, Kuo and Ford have constructed a dimensionless quantity, whose magnitude reflects the amount of fluctuation in the back-reaction term and hence on the validity of the semiclassical theory. In this paper, we evaluate this quantity for the minisuperspace model of a quantized massless scalar field in a Friedmann universe. We conclude that the semiclassical theory for the model we consider here can be relied upon only if the scalar field is in states like coherent states. The implications of our investigation on the complete field theory are also discussed.