I got my 6600k to 4.6ghz at 1.365v and tried to get it to 4.7ghz at 1.4v, but it would not have it and BSOD. So I was to worried to push past 1.4v and did not bother, but with the Noctua nh-d15 I only hit 49c under stress.
Now as for long term use with 4.6ghz I had to run it at 1.365v, but if I come down to 4.5ghz I only needed 1.285v for a stable OC. And that is a massive difference for only a small gain!
But at the end of the day its up to you if you think its worth it or not.