I can't see anyone objecting, so long as you get your job done by the end of the day. It's only a problem if you're wasting money by utilising bandwidth that's required for another purpose or that costs money e.g. a dial-up account.

I use the net all the time at work, and my employer doesn't care, especially as it's usually during my lunch hour anyway. So long as I get my job done it doesn't make any difference. It's kind of a part-payment for my services, although this would obviously not apply in all cases.