My opinion is that it is accorded to the length of time the mod was in usage. For example today I would not think to ask Black Phantom or Darkloke permission to use their 1.08 patch/WoW creatures because they are since long time in common public usage. Of course Darkloke will always have credits for his creatures in a read me.

2 years ago I would ask Darkloke for permission, but not today, when creatures were already used in several maps or mods and became part of Heroes graphics.

But if someone is going to make a mod now, and I take it, I will always ask permission. Courtesy between heroes fans and modders is a must because when someone is interested and asks for your work, it helps keeping enthusiasm for doing more.