Well, it's usually best to get a later model, which in the case of the Nexus 7 would mean a serial starting with C10 or C11 (October or November manufacturing date). However, that's not a guarantee of getting a defect free model. It's also no guarantee that you'd get a defective unit if it was a C06. Regardless of the seeming panic in the forums regarding defective units, the vast majority of earlier builds were defect free. But my guess is that units built later are even less likely to have defects, simply because of manufacturing adjustments made due to the earlier reported defects.
In other words, don't sweat it too much. If a later model is available then grab it, and if not then don't worry about it. Either way, when you open it, inspect it for screen lift and dead pixels, listen to the speaker at high volume to make sure it's not blown, and set it up and take it for a quick (figurative) test drive. If anything is wrong, exchange it!
Sent from my Nexus 7 using Android Central Forums