If you bought it recently then its almost certainly the G0 stepping, which runs cooler and is better for overclocking.
I guess I'm still thinking back to when I got mine three or so years ago and it was just as they released the G0 stepping, so all the shops were trying to quickly sell off the B3s before anyone realised. I think I shop I got my G0 from was the only one that would guarantee to send you the G0 - nowhere else would make any promises about which one! I suppose if you have a few hundred near-obsolete CPUs left to get rid of, you're gonna do what you can to shift the stock! A few weeks later and that had died down though...
Anyway, no, the Q6600 is a foolproof overclock so long as you have a half-decent motherboard and some decent airflow inside your case. You can even safely get away with the standard Intel cooler for 'mild' overclocks, though a decent HSF (I have a Noctua one which has been really good) will run quieter and cooler.
I've run mine at 3ghz ever since I built the PC a few years ago - I don't think its ever run at stock clock speed apart from the first boot into the BIOS! Has been a "set and forget" job where I don't even remember about it being overclocked. A few months ago I knocked it up to 3.2ghz and the temps didn't noticeably rise. Never had any stability problems at either clock speed, and its crashed about a grand total of two times in the 3 years I've had it!