Drift-reduced plasma fluid models are commonly used in plasma physics for analytic studies and simulations, so the validity of such models must be verified for the regions of parameter space in which tokamak plasmas exist. By deriving and comparing the linear dispersion relations for the drift-wave instability for both a drift-reduced model and a full-velocity model, the importance of the physics lost with the drift-reduction is examined. This analysis is generalised for typical tokamak parameter spaces and is then applied directly to JET data. It is found that drift-reduced models are generally more applicable to the edge plasma (<10% error), while the core plasma shows more significant disagreement (>30% error) particularly at mid-radius. The effect of drift-wave mode number and wavelength also play a key role in determining the accuracy of drift-reduced models.