I have DEM data that is in a GCS (NAD 1983 (CSRS)) with vertical units in meters. My understanding is that before doing any processing on a raster, the horizontal and vertical units should be made to match, so the DEM should be projected into a PCS with units in meters as well. However, based on my reading this applies mainly when you want derivatives of the elevation data (i.e. slope, aspect).
If I wanted to compute zonal statistics on a DEM, is it considered best practice to first project the data or to compute the zonal statistics directly on the raw data despite a mismatch in projections?