The introductory notes to the section on imperatives give the form γράφω - γράφε - γράφετε. I assume that this is just a mistake and the imperative forms are always from the perfective/aorist stem. That seems to be the case for the other examples given in the introduction.
Here's another source: http://www.greekgrammar.eu/pdffiles/imper.pdf Apparently you'd use the present tense form (φράφε / γράφετε) if you want to express continuation (be writing!). You'd use the aorist form if you are not interested in the continuation of the task (γράψε / γράψετε = write).