Turning those features off can provide stability, but that's in cases of really high overclocks. If your systems isn't stable with small overclocks, then something isn't right.
With something as mild (for the E8400) as a 600MHz bump, as you pointed out, the core will handle it with ease. Yes, if you leave it on auto voltage will be adjusted, up and down, but not more than the chip's specs list as safe, which is between 0.85 and 1.3625 volts.
If you only have stock cooling, you shouldn't be overclocking anyway. Yes, lol, keeping Speedstep on isn't going to save you as much money as never turning on your air conditioner
, but, over time, the money is going to start to add up.