It works best, just a rule of thumb and a consensus between ocers, I would say. It may work the other way as well, but for full stability in 2d and 3d I had to set equal values for both. The first screen settings worked well for prime an linx, but 3d seemed a bit unstable. I then noticed I had not increased qpi pll and catched up on setting it to IOH level. Worked!

/edit. What do you need 1.3V QPI PLL for?? I needed 1.3 - 1.35 for 235MHz bclk.