If your overclock needs more than stock voltage to be stable, then leaving those options enabled can cause instability due to voltage sags and spikes at the wrong times. Here's why:
Those two features save power and energy by changing both the multiplier and voltage provided based on CPU load. The problem becomes this: those features are reactive, not proactive. Thus, the CPU has to already be at full load for C1E to turn up the voltage and multiplier. The problem now becomes a massive voltage overshoot (spike) as the MOSFETs need to suddenly deliver significantly more power in the span of ~100us.
On stock voltage, the voltage overshoot is purposely countered by vDroop and the processors are specced to be stable and reliable during that spike. On over-stock voltage, the overshoot can become quite large, especially when combined with overclocking as the power draw becomes quite significant.
It basically boils down to this: the higher your CPU's power draw, the more possibility for damage to the processor by way of voltage overshoot when C1E decides to step in and turn up the speed.
If you want to save power, don't overclockIf you want to overclock, don't fool yourself into thinking you'll be better off by leaving those enabled. And if you disable those features, you should also see if your motherboard supports some form of vDroop Disable (some call it loadline calibration).
I guess it depends on the system. I run mine with C1E and Speedstep @ 3.66 w/ no problems at all. In fact I like to see the speed changes as I run different applications. Its quite a sight.
