[R-sig-Geo] is raster terrain accurate for WGS84 tiles?

[Stanley, Thomas A. (GSFC-617.0)[USRA]]

At https://rpubs.com/ajlyons/rspatialdata, a presentation shows the raster package's terrain function used directly on an unprojected raster near the Equator. I'd love to skip the steps of projecting and unprojecting my DEM tiles (eg http://www.viewfinderpanoramas.org/dem3/M11.zip), but I'm not sure the function is intended to be used in WGS 84 for all locations. The output values certainly look good. Can I correctly calculate slope without projecting my tiles, or is that wishful thinking?

-Thomas

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