The assumption for the earth's gravity coefficient is 9.81m/s² in terms of sensitivity error. The actual earth's gravity in a specific location does not affect sensitivity error.
For the offset, indeed the local Earth's gravity does have an impact. but this offset is quite limited, about 0.5% (50mg) between the min and max value on Earth. Considering that the 0-g offset after soldering alone is typically +-80mg, without considering other factors such as cross-axis alignment, temperature and mechanical stress.
In general, for applications using accelerometer data only use a simple offset calibration which combines a wide variety of factors that impact the offset, and do not calibrate for sensitivity, cross-axis effect, or temperature.