Hmm oddly I've never heard of this rule before. In fact my 1156 socket system doesn't even have this option. I guess it must be a 1366 thing. I've only got Vcore, IMC/Vtt, DRAM, PLL, PCH voltages.
Anyway, doesn't seem to make sense that you need a certain ratio or gap in the voltages... I would think that as long as you don't reach the maximum spec for a voltage you're ok.
Asus calls QPI vtt all it is is the bus between the memory controller and the memory and really from what I have heard is you can run higher than 1.65v dram if the QPI/vtt is within .5v of dram voltage it sounds wierd to me to. I have seen this in numerous threads on several different sites, but I wont try it, and I wouldnt advise anyone else to either!
That's worth noting, I'm wondering if that's the reason at 178 base clock my available ram drops from 8gb to 2.4gb... Hmmm! Well it would be very interesting if a slight bumpt to IMC will make it work again - the RAM was fine for a week or so. Can't remember if I tried lowering voltage but as I recall I didn't touch it, however, I did have the DRAM voltage a bit higher at 1.68 I think because the RAM was slightly overclocked.