If one is proposing to use someone else's original work, it's a simple act of courtesy (if nothing else) to ask for permission before using it, even if it doesn't seem to be copyrighted. At minimum, one ought to acknowledge the source. The case cited re: Jag is an example where hurt feelings likely could have been avoided if Jag had spent the 30 seconds or so to simply send off a quick email first, or if he'd simply stated "this information came from ...". These are such simple steps that it is hard to see an argument against doing them.

In some -- perhaps many -- cases you might not get a response to a request, or the material in question actually belongs to a third party, so a bit more work is needed in those situations. But I strongly feel that one is obligated to make an effort. Again, at minimum, simply stating where you got the material can go a long way to preventing misunderstandings and hurt feelings. Seems like a good investment.