Well I've heard that 2600k has been pushed to a stable 6.1GHz in one case (obviously with crazy cooling.)
The answer depends on your situation. Obviously it is a very fast CPU already, and if you don't have money to spend replacing parts or upgrading cooling, or if you don't have the time to be dealing with potential problems, then there isn't any real need, as 2600k is already a great CPU.
However, pushing the clocks higher will obviously equate to higher performance. It is suggested among reviewers that the primary reason to go for the cheaper SB CPUs is that they can then be OCed to compete with the more expensive 1336 CPUs.
Personally, I won't be OCing mine. But I don't have money to spend on replacing it if/when it all goes balls up.
Whatever you decide, just remember that it is already a great CPU, especially for the price