I don't know the nature of WYs work but think that my feelings on the issue would be determined by just that.......
If there was a 'set amount' of work to be achieved daily, I would be inclined to allow internet usage providing all of my targets were being met. If I felt that the time used on browsing was such that my employees could be reasonably expected to increase their output, I'd raise my targets.
If my employees were tied to the PC and only worked as and when presented with work ('phonecalls and such) I'd be content to let them while away the boring interludes by surfing, providing all the work was dealt with in good order and time as and when it arose.
If there was a steady turnover of work, I wouldn't object to the occasional 10 minutes throughout the day, again, providing *my* business took priority.