I think it's impossible for an announced raid to happen 5 minutes before server save. Also, playing before server save for months and talking with many other people, we think that it is impossible for a raid to start ~1 hour before server save.
So, what is the closest time to server save have you seen a raid being announced?
This info is important because rare bosses use the same raid mechanics, the difference is that most of them are not announced. I think that if you couldn't find a boss 1 hour before server save, it is not worth to check again 10 minutes before server save.