A comparison of the Casimir force theory and experiments with Si rectangular gratings is performed. We use Rayleigh decompositions to evaluate the Casimir force in Si grating - Au sphere systems for two Si grating samples. Ratios of exact Casimir force to proximity force approximation (PFA) and pairwise additive approximation (PAA) results for the Casimir force are found. The difference in the use of theoretical PFA results and PAA results extracted from experimental data is emphasized.