I think if something is popular enough, you don't really have to ask.
For example, if every person who made a movie based on mario and used him as a sprite waited for nintendo to respond, then holy shit.
But say If I made a custom sprite for cavestory, and someone wanted to use him as an npc or something like that (I dunno im asshating this) I would like to be asked since, after all, it's my design and I have the right to not let others use it. Since we all know if I made something it wouldn't be hard to ask since my reply would be swift since I got no one askin me.
But if you were askin on somethin that was globally known, then you really don't need to ask.
However, all in all, credit must be given. No matter what. Without giving credit your stating that you created said piece. In doing so your stealing that thing your using, and that is illegal you know.