I would say that it depends upon the intended use of the image, and whether said cropping significantly changes the viewer's perception of the scene. In an "artistic" image, cropping is perfectly acceptable. In a "news" image cropping must be done so that any portion of the original image that is cropped out does not alter the meaning of the image. Take for example an image of Governor signing a bill into law. Gathered around the Governor at the signing are a Republican and a Democrat that co-sponsored the bill. If you were to crop the image so that, in addition to the Governor, only the Republican remains, the perception of the event has been altered.