The interpretation and comparison of viscosity data for fluids in planar Couette how are discussed using a generic rheological function, familiar from studies of polymeric liquids. The particular data are generated by non-equilibrium molecular dynamics (NEMD) simulations of model fluids, however the analysis has wider validity. The mathematical properties of the established flow curves are investigated and, inter alia, a consistent definition of the critical shear rate is established. It is also shown that the use of inappropriate, but commonly used, independent variables leads to curves whose qualitative behaviour is extremely sensitive to the physical parameters. Such a range of qualitative behaviour can lead to erroneous quantitative conclusions when interpreting physical (or model) data. A particular example is the assumption of a linear dependence of the viscosity on the square-root of the shear rate, as often used in the NEMD literature. The developed framework also enables the systematic comparison of simulations of the same molecular fluid performed at different state points. This is illustrated for the case of three recently published and apparently conflicting NEMD results for n-butane.