Never at the office which is a network.
Even though our IT guy is very cool, and we even exchange "humourous" emails, I wouldn't surf terb at work.
That's why i have the internet at home.
If i am travelling for work, i have a lap top with a dial up service, so that is safe since there is no way for them to trace my activities without perhaps calling telus, but that would be a little over the top.
The only thing i ever go on from the office is maybe the globe and mail, or canoe, maybe a finance site. They don't care about that and no tongues will wag.