I just use powered risers on every card exclusively.  I make them myself so it's really no big deal or extra cost.  But if you run more than 3 power hungry cards I would at least use powered risers on the rest as it creates too much load on the mobo.

I run all the powered risers off the same PSU that the mobo is plugged into.  The same way it would be if you weren't using powered risers, the power would all come through the mobo.  I've tried separating them before and it can at times create issues.

Depending on the cards you use figure about 50w +/- per powered riser.  so account that 250w for 5 cards for the main psu.  That is with 7970s/280x's.  R9 290x's and 7990s the power draw could likely be more around 75w+ per riser.

At the same time you can subtract that 50w from each card for the psu power requirement