Reynolds-Averaged Navier-Stokes calculations of turbulent premixed and stratified flames in two different configurations are performed to assess the effect of small-scale mixing rate models. The small-scale mixing rate is commonly known as the scalar dissipation rate. The Libby-Williams model involving delta functions is used to calculate the mean reaction rate. The classical model for the scalar dissipation rate underpredicts the mixing rate and gives poor agreement with mean values of velocity, temperature, and species mass fractions. Two other recently proposed dissipation rate models give good predictions at various stratification levels. Further analysis shows that despite adequate mean predictions, there are substantial differences in the mixing rates, fuel mass fraction variance, turbulence kinetic energy, and turbulence frequency predicted by these models. Additionally, the turbulence quantities predicted by the models show opposite trends with increasing stratification. Further Direct Numerical Simulation or experimental data is required for a thorough understanding of these differences.