We investigate nonresonant contributions to Rayleigh scattering cross sections in atomic transitions. The problematic nonresonant contributions set a limit to the accuracy to which atomic spectra determine energy levels. Specifically, we show that off-resonant contributions for the 1S-2S two-photon transition in atomic hydrogen, which is one of the most accurately measured optical transitions today, are negligible at current and projected levels of experimental accuracy. The possibility of a differential measurement for the detection of off-resonant effects in one-photon transitions in atomic hydrogen is discussed.