The disks of spiral galaxies contain global magnetic fields that play an important part in regulating star formation and the exchange of matter between the disk and the halo, or the surrounding intergalactic medium. Despite their importance to the evolution of galaxies and an increasing number of detailed case studies, many questions about magnetic fields in spiral galaxies remain unresolved. Certain questions such as the relation of the magnetic field to properties of the galaxy as a whole such as the star formation rate, and the environment of a galaxy require substantial samples with well-defined selection criteria. For an observational approach to the evolution of magnetic fields in galaxies, the key question is what we can observe for representative samples of galaxies at low and high redshift, independent of resolution. The polarization of unresolved spiral galaxies allows a statistical analysis of magnetic fields for samples of galaxies at any redshift. I will discuss how deep polarimetric observations combined with shallower wide-field polarization surveys such as GALFACTS can be combined to provide new insights into the evolution of magnetic fields in spiral galaxies.