I am in the process of redoing my OC, and finding my minimum safe voltages for my current OC on my i7 970 Extreme edition CPU.
 
For 24/7 use all Ive done is bump the Multi to 29 as that proved to be the highest I was comfortable running with the temps it would reach on air at max load with the voltage needed to run it. If I'm not overclocking the QPI at all, should I even raise the VTT at all? Ive usually had it at +175, but have been testing it with +0 (or whatever is the lowest amount added, I'm at work now) and it seems to run fine, but more testing needs to be done.
 
VTT is what is used to stabilize the QPI, and you raise the voltage when overclocking it, so if your just using the multi it shouldn't be raised at all right?

With the VTT part out of the way all thats left is finding the minimum vcore, and I think raising the DIMM voltage up a bit, I had it at 1.62 as the bios always showed it running a bit higher voltage then what it was set at, and 1.62 made it say 1.65 but from what ive been reading lately it seems what it reports isnt that accurate, and what it is set to is almost exactly what it is running as. Ever so slightly undervolted ram would explain once in a blue moon lockups or errors running IBT or Prime95. (Like IBT running 50 passes on xtreme mode, then finding an error on after pass 20 the next run, etc)

CPU VTT is for the "Uncore" portion of the CPU, the main components of the Uncore include the Integrated Memory Controller (IMC), 8MB L3 Cache (not the L1 data/inst or L2 cache's) and the QPI link.
 
Mostly raising CPU VTT is usually necessary when using higher speed memory (DDR3 1600 and above, the Nehalem based i7's are only officially rated by Intel to support DDR3 1066 memory) as this will also increase the IMC frequency (IMC freq must be 2x the Memory frequency on the Nehalem based CPU's but is slightly less on the Westmere based CPU's 1.5x or 1.8x, thats just a guess but gives you an idea, the IMC will still get overclocked when the memory frequency is increased just not as much as the older Nehalem based CPU's).
 
The 8MB L3 Cache will run at the same speed as the IMC as they share the same multiplier, so again this makes CPU VTT more important when running the memory at a higher speed.
 
IMC frequency is shown as NB Frequency in Eleet and CPU-Z (Just a throwback to the days when the Memory Controller was in the  North Bridge chip).
 
In my experience you need to set a pretty high QPI speed for it to be necessary to raise the CPU VTT voltage  to stabilize it.  The QPI Link has a separate multiplier to the rest of the "Uncore" and seems to be the least necessary for CPU VTT voltage increases.
 
CPU VTT should always be within 0.5v of DRAM (Memory) Voltage, so if you have a voltage of 1.65v for the DRAM then the CPU VTT should be atleast 1.15v, this is ok on the Extremes because their default CPU VTT is 1.20v but for those with a 920 or a non extreme I7 then an increase of 0.05v (+50Mv) should be mandatory when running a 1.65v DRAM voltage regardless of the IMC/Memory frequency.