I wouldn't. I don't think it's at all necessary.
It's not as if you would be stealing views from them or anything.
The only reasons I would ask permission for is
*if you wanted to download the video, do some editing to it, and re upload it
*Or if you plan on using the video to make a profit of some sort.
You don't have to ask permission. If they would strongly like to block people from uploading their work elsewhere, they can disable embedding, which means the video can only been seen on youtube/wherever you found it. They can also leave a message asking people not to copy it in the description.
I'd still give credit in the description or title here, though.
