If the computer idles down it should run at 1.7 GHz which will lower power in that state. If you're running something like Folding 24/7 go with the 1.37 and try to keep temps below 65C. Max operating temp 72.6C per Intel.
Ok, I see. I have Corsair H100 watercooling, so it's pretty cool all the time.
A friend of mine said that 1.52v was the max recommended when doing benchmarks, and that 1.38v was the max recommended for 24/7 use.