I think it is an accepted practice among renderers to adjust the shadows so they help present the design. That was the same for hand-constructed perspectives as it is today with computer-generated. The goal typically is to make the design look as perfect as possible. Your typical rendering isn't considered, at least by those making them, as a "shadow study". I think most people making renderings do not consider such idealizations to be unethical. (They would likely agree it is unethical to labelled the rendering "July 4th" if the shadows are based on March 2nd). Other rendering 'enhancements' relate to the reflectivity of the glass, not illustrating power poles, wires, garbage dumpsters, the reality of the huge parking lot, etc.
Presenting a real shadow study at a public meeting can be the curse of death to almost any project. People live much of their lives in shadow without even realizing it or having any concern. Yet when they see an animated shadow study, of the kind you can do with a SketchUp model, they get very concerned. Overly concerned in my opinion. The shadow study is presenting information they aren't used to seeing and are unable to evaluate objectively. Neighbours arrive at the public meeting thinking they have to protect themselves from being affected by the proposed project. Seeing that a shadow may fall on the sidewalk in front of the building can be enough to trigger the "you see? It will impact on me - I walk there everyday" even though their own property development is doing exactly the same thing. Once they perceive "it affects me negatively" there is little hope that you can convince them to support the project as self-interest and resistance to change are powerful forces. Unfortunately, in the real world, protecting self-interest trumps fairness.
Regards, Ross