Given that Chrome and Safari use webkit has anyone yet found anything that renders differently on Chrome than Safari? Is there any reason at the moment to test sites on both, o
Part of this is knowing what is dependent on the rendering engine and what isn't. Javascript, for example, is handled differently in both browsers (google has their own custom javascript renderer), so if your page uses javascript substantially I'd test it in both.
This is probably a good place to note that Chrome has been added to BrowserShots so you don't even need to have it installed to test on it and Safari.