Image ratings help users organize and search through the various levels of explicit content contained on Danbooru. It's very important to rate your uploads properly. By doing so you help ensure that searches continue to return accurate results.
Do note that while Safe pictures MAY have a butt and such, that's more or less so that a picture that has only a little tiny bit won't be disqualified on principle, like post #2764761. A picture that focuses on the sexy aspects, such as this, is Questionable.