I did it for the first time on my last long form video even though some gurus say to do it first (otherwise how do we really know what to make the video about? or something like that lol).
Did it make any substantial difference to how I filmed my video compared to if I did it afterwards? Maybe because the idea was solid in my head: this is about Leanne going viral off her first video this whole video hinges around her achievement and how I think she did it.
Have you ever made the thumbnail before you've filmed the video?