I would think you wouldn't need powered risers for the x16 slots
I have a board with 4 x16 slots, and have been using unpowered risers for them for over a month now with no issues. (3x r9 280x power hogs)
I figure if the board was designed to power that many vid cards, there isn't a need for powered risers.

now for the x4 and x1, definitely use powered, I don't think the mobo can push enough power to those slots to power the cards properly..
but that is just my experience, i'm sure someone who knows better will be along to help out more.