My philosophy on this is like much else, as long as you're getting your work done without skidding in at the deadline, getting your work done well, not interfering with anyone else's ability to get their work done, and not making the company look bad I wouldn't care what they did with their time. They could go home and drink two 12-packs every night as long as they showed up sober in the mornings and didn't go out making trouble for others at night.
Now, the first person that didn't get his work done because he'd spent too much time on IM or something would wish he'd never been born...
ZV