I feel that even though "publishing a negative" has such a bad reputation and is thought to be a waste of effort, that a number of factors can mitigate that; really it depends on how you sell your story, and the general importance of the research in your field. It's important to plan an experiment where no matter the outcome, the results are interesting/publishable. Of course it's often not feasible, but when it is it's a great experiment, because you know that no matter the outcome it's still a productive use of time.