OC is a hobby, not a necessity. This is true of new Intel cpu's in particular in a general sense. If you have an older gen, like 1st gen Sandy-Bridge, then some OC can bring you closer to newer skylake standards, which is where you'll see any equitable performance in benchmarks. But thats Intel, at normal usage. Using high end gpus in sli at 4k is different, OC is generally warranted to get better fps. If you plan on using high tier ram, like 3200MHz, there's a good chance you'll need some OC to get performance and stability.
AMD FX is different. AMD single thread performance is only @ ⅔ that of Intel, clock for clock, so to keep up with the bigger Intel boys and their toys, it's advisable to OC the FX, which sees greater results than Intel, clock for clock. Whereas with Intel avg you may see a 5fps gain, the FX is capable of 10fps or better. (which in most games only brings it up roughly to Intel i5 fps anyways)
So, the i7 6700k, OC if you want, generally it's more than enough to run anything at stock clocks, just make sure to buy an appropriate cooler as the 2 k cpus don't have stock coolers included (Intel finally got smart, included coolers are wasted on a cpu most will toss simply because you'll need aftermarket to oc)