Its recomended to stay at 1.4v or under, for 24/7 use, that being said i used to give my old E5200 cpu 1.45v 24/7 as its such a cheap cpu to replace, i was'nt to bothered.
Main thing is that your cooling is upto it, im sure others may agree, or some will say to def stay under 1.4, but best to wait to see what the majority think.