I don't know about a general policy, but mine is that if an artist has a notice saying they're happy for their work to be used I'm okay with that and I'll credit them, if they say they're not happy with it I respect that, and if you're not sure just ask them. Most artists like being told you loved their work enough that you'd like it as an icon, even if the answer is no, and if you say that you'll credit them they're usually happy for you to go ahead.