I've recently started using the Pomodoro Technique (which was linked to on HN a few months back), and I've found tracking the time I spend very helpful for my self-discipline--preventing myself from getting into that fugue state where I'm looking at the browser, going click, click, click, click, and then realizing that six hours have passed and I've accomplished nothing.

But I absolutely would not want my manager to use this kind of information to evaluate my performance.