Really! Why bother?
The only reason to add any visual effect to a data visualisation is to aid interpretation or the understanding of data. A drop shadow does not in anyway achieve this - in fact, it quite often achieves the effect of misleading the viewer as the eye is drawn to the area below the data series and not to the data points themselves.
Features like these should be hidden from the amateur data analyst and only used with extreme caution!
A new and really cool chart feature that we introduced in 7.0 is the capability to add drop shadows. You can now enhance the look of the chart to simulate 3D depth for some really eye catching charts. I find myself using this new param all the time in order to give my charts a little more depth.