Should I get a powered riser or non-powered riser?

[Shadow Bullet]

I have a EVGA GTX 970 FTW hooked up to a corsair RM series power supply. If I am getting a riser does it need to be powered or not?

 

[cin19]

If you have only one gtx970 it don't matter, if you have more than two, you are better to get the powered riser.