coordinate system - More accurate way to calculate area of rasters

In my daily work, I am constantly asked to calculate areas of global raster datasets in geographic projection at 30 arc second resolution. These datasets are normally the result of a Combine operation (a typical example is a vegetation classes combined with country layer). To do this, our unit created a raster dataset with the area of each pixel in geographic projection at 30 arc second. With this area grid, a zonalstat is performed to sum the areas for each class. Since I am not sure how this area grid was created, I always wondered whether this approach is more accurate of just reprojecting the raster in a equal-area projection (from simple tests the results of the two methods are similar). Does anybody experienced a similar situation?