(FORTUNE Magazine) – I'm terrible at managing up. At worst, I've seen my bosses as a distraction from getting my job done. At best, they were allies to run interference with their own bosses. So I've never had trouble telling the boss what I thought of him. Often as not, I regretted being so frank. Bosses can only take so much bluntness before they either shut you out or start thinking about ways to get rid of you.

All of this begs the question: How honest should you be when reviewing your boss? Don't be so sure you won't be asked. Fully 32% of organizations surveyed last year by the Society for Human Resource Management use 360-degree feedback, in which workers are asked to appraise their supervisors' performance at review time. (These reviews are also known as subordinate appraisals of managers, or SAMs, perhaps to ensure that you don't lose sight of the fact that you are still the subordinate.)

So how honest should you be? The short answer: very. For one thing, lying is generally bad, as your mother doubtless told you. But in upward reviews, it can wreak particular havoc. Your boss might, for example, be a notoriously bad manager who also happens to be spiteful. You fear retribution, so you give him exemplary comments across the board. The result is that your boss' boss thinks he's doing a fine job. Essentially, you've put your stamp of approval on his behavior--and ensured that he will live to torture again.

However, there is wisdom in taking a measured approach to your negative assessments. If the boss is unclear in assigning tasks or reluctant to delegate, say so but leave it at that. There's no need to embellish, much less to treat the upward review as your opportunity to get back at the boss for all those times he fell short. Be clear, honest, and fair--just as you wish any boss would be with you. And if the guy hunts you down with an ax (metaphorically speaking, of course) after hearing your feedback, so what? If he's that awful, you'd be better off working for someone else anyway.

As for me, I'm still not great at managing up, but I'm getting better. Of course, that may be more a reflection on my new boss than it is on any behavioral adjustment of my own. And I'd point out his copious strengths on my upward evaluation...if only we used them.

Jeffrey L. Seglin is the author of The Good, the Bad, and Your Business: Choosing Right When Ethical Dilemmas Pull You Apart. He teaches at Emerson College in Boston. He can be reached at jseglin@post.harvard.edu.