Depends if you're being paid for your attendance or to perform a job. If you're paid for attendance (and if you test most bosses' expectations, that's where they sit) then any distraction is a no-no (and, not coincidentally, your long term desire to work there may also be a no-no).

If you're being paid to perform duties or hit milestones, then that's more important than whether you are focussed only on your work when you're at work. A needed break or relevant distraction like HN shouldn't be a problem.

I use the Alt-Tab rule when making this decision myself. If one of my team mates walked past, would I Alt-Tab away from this screen or would I be fine to let them see what I was doing?