Thinking about it more, the cards need 12V from both end so I don't think you need a 5V rail at all.
In fact a PCI-E slot doesn't provide 5V, only 12V and 3.3V.

The USB riser might feed itself from the PCI-E lane (3.3V) and not the 5V from the molex. Not sure though.

Edit: tried a riser with the 5V rail on the molex disconnected and it didn't work. So the USB riser does require 5v.