Or should they write about what they love? Paul Horwitz has some wisdom:

At least as far as tenure is concerned, I just don’t think the numbers warrant undue concern; and even if they do on the margins, I think too many junior faculty are too worried about tenure, not just reasonably concerned but concerned out of proportion to the facts.  And I worry that the more worried they are, and the more strategic they become, the more distorting effects that will have on their happiness, their well-being–and their own actions when they become senior faculty and are judging their juniors.  Given the relatively low risks involved, I think it is better to (mostly) foreswear strategy–kow-towing, putting your senior colleagues’ tangentially related work in the footnotes, etc.–and focus just on what you want to achieve as a scholar.  Without being too naive about it, I ask my students to focus on doing the right thing as lawyers, even if there are professional costs involved; can I ask any less of myself or my colleagues?

I made this decision a few years ago. I wanted to write about what I love and what I can produce excellent scholarship–not about what I thought would guarantee me a job teaching. Perhaps I am paying the price for that now, though, in all honesty, if I had secured a job in an area such as tax or bankruptcy, I may as well not have taken the job in the first place. I would not be a productive scholar, I would not enjoy teaching it, and I would not be able to bring merit and success to the field.