Measuring accurate radial velocities of rotating early-type (O-B-A) stars using cross-correlation techniques is hampered by the fact that object-template spectrum mismatch causes systematic errors that do not cancel out sufficiently in these spectra. This series of papers aims at quantifying those mismatch errors, understanding their astrophysical origin, and developing strategies to avoid them maximally. In this first paper, we employ synthetic spectra to study mismatch between A-type main-sequence stars caused solely by differences in T_eff and log g. We show that this spectral-type mismatch varies greatly throughout the spectrum, though with some degree of systematic dependence on rotational velocity and, to a lesser degree, on temperature. We propose a scheme for selecting spectral regions that should provide, for main-sequence A-type stars, accuracies better than 1 km s-1 for v sin i