Full pattern fitting is one of the most accurate and precise methods available for determination of lattice constants, at least amongst techniques that are routinely available. Lattice constant determination is one of the few areas where powder diffraction routinely exceeds single-crystal measurements. Between powder techniques, high resolution synchrotron measurements have probably the greatest precision, though neutron measurements, with their improved sensitivity for high Q scattering, may sometimes be even better, but it should be noted that when it comes to accuracy, neither has the advantage that laboratory instruments have in that the characteristic x-ray wavelengths have been standardized to much better accuracy than can be achieved for calibration of a neutron or synchrotron beamline. While I do not normally recommend use of an internal standard in powder diffraction, if obtaining the ultimate in precision and accuracy is desired, use of a high-resolution instrument at either a synchrotron or neutron source with a primary standard integrated into the sample would likely provide the best compromise between precision and accuracy.
It should be noted that if an accurate structural model is not available for the material where lattice parameters are to be determined, a traditional Rietveld fit may not perform as well as desired, as the intensity mismatch could perturb the lattice fit. Use of a Pawley or Le Bail fit (see ยง20.4), will avoid that problem.