Intel's Turbo Boost doesn't mess with the BCLK. It adjusts the multiplier to overclock. And the more cores being used, the lower the max multiplier it will allow (4x for 1 core (for a clock speed of 3.7Ghz) down to 1x (3.4Ghz) for 4 cores).
It's very safe to use. And since it's Intel's tech and part of the specifications for the CPU, even if it does by some miracle happen to fry the CPU, it's completely and undeniably covered by the Warranty whereas, even with the 2500/2600k's, damage caused by overclocking
is not covered (see footnote 1: "Intel has not tested, and does not warranty, the operation of the processor beyond its specifications.")
Not that I've ever heard of Intel refusing an RMA because of OCing damage, but I prefer not to risk it.