The Burra Charter underpins the "official" view of this issue -
3.2 Changes to a place should not distort the physical or other evidence it provides, nor be based on conjecture.
and the ASLaN Charter is the most prominent "unofficial" view of the issue (and has long been good enough for most people, including most pagans!) -
Please take care when visiting sacred sites to leave them as the next visitor would like to find them.
It always defeats me trying to understand how anyone can think leaving their own mark at a place after they leave and have gone home for a bacon sandwich can possibly be seen as fair on everyone else that comes along!