We examine phase diagrams of model binary polymer blends and estimate the temperature range where mean field theory is applicable. The computations use both. compressible and incompressible Flory-Huggins theory and the corresponding random phase approximations. The size of the nonclassical:regime is estimated in terms of the Ginzburg number Gi for both the one and two phase regions. Model calculations indicate that compressibility (and thus pressure) can significantly affect widths of the nonclassical region and critical temperatures (T-crit) for phase separations. The qualitative scaling of T-crit and Gi (and hence the widths of the nonclassical regime) also differs between compressible and incompressible blends. Incompressible symmetric blends with polymerization index N yield constant values for both the product NGi and the ratio T-crit/N, while compressible polymer mixtures produce NGi as nonconstant and even possibly as varying nonmonotonically with chain length. Pressure and entropic interaction variations are shown to change miscibility limits and strongly to affect Ginzburg numbers for some blends. Model calculations provide insight into common experimental methods for determining the onset of the nonclassical domain.