In posts like this, and from other sources, I have heard that the handset should be fully charged before flashing or installing a customer ROM. I assume this is because the calibration for the battery is kept somewhere within the ROM, but I don't know for sure. Is this really necessary?
It's just because, under some circumstances, flashing can take a while. You really don't want your device shutting off while it's being flashed; it can brick the device.
The calibration for the battery is stored, yes, but usually it gets adjusted within a few charges of the phone. It might be a little inaccurate for a while, but it's not a major concern.