MarkT:
Looking at the spec sheets recently it says the RC oscillator is +/-10% as it comes from the factory, yet can reach +/-1% after user calibration. I think that means that factory calibration consists of writing the same value of OSCCAL into every chip on the wafer!!
I have four ATmega328Ps running with the internal RC oscillator at 8MHz that seem to have no problem doing serial communication at 9600bps with whatever calibration comes from the factory. YMMV, I could very well just have a "good" batch. I'm just doing output, Serial.print() debug and informational stuff, but that seems to work fine.
Your post made me curious, so I checked OSCCAL and found all four were different, 0x9B, 0x9E, 0xA7, 0xB3. These chips were all from a single order from Mouser, they have the same date codes and all, but I wouldn't say that means that they're all from the same wafer.