CPU VTT: It is a Crucial voltage when pushing the Host clock, the Uncore Frequency, and the Memory Frequency. Default voltage will give around 1.165v. A good setting lies in the region of +250mv to +400mv. (The higher you push Uncore Frequency and Memory Frequency, the more you need typically)
CPU PLL VCore: No help of much, usually can just ignore and leave at Auto. Or, even lowering this can be better.
IOH PLL VCore: On other X58 boards it is synched with CPU PLL VCore, but it is separately adjustable here. Not much help usually, can just leave at Auto.
QPI PLL VCore: Can be more useful towards the high speeds of QPI Link (4.8GT/s, 5.6GT/s, 6.4GT/s). I usually Synchronize with IOH Vcore. I usually use 1.40 to 1.45v.