Doc students often ask when they should start publishing. One school of thought is to wait until their dissertations are finished. According to this logic the dissertation should hold the totality of the doctoral candidate's focus; anything that gets in the way of this focus is seen as problematic.
I don't agree with this for most people (of course, there are always exceptions) for a few reasons.
1. Writing begets more writing. Using article writing as a warm up is a great way to "enter" your dissertation. The more consistently you write, the easier it is to be productive.
2. When else are you going to learn how to write articles but now?
3. You can bet that you will be competing for faculty positions with other doctoral students with publications. I have served on more than my share of search committees- publications are a key to rising to the top of some very large stacks of applications.
4. Do you really want to start a tenure track position without articles in review and a couple nearly done? I have seen that the undoing of many a new assistant professor.
If you are a doc student, pick an article to write today. Choose a journal. Make it a goal to have it done within three months. If the average article is about 20 pages, with references, than that means less than a quarter a page, or about 70 words, a day.