Ethanol Content Sensor 0v or chassis ground

One data sheet I've found says "battery ground", the other "sensor ground". For this somewhat unique type of sensor, don't see any concern from using a chassis ground. What are peoples experience and/or preference?

Since it's the waveform frequency and pulse width which represents the important information. As I understand, even if there was a small ground offset, this would not effect the decoded value at the ECU.

You're right in so much as the square wave signal from the ethanol content sensor is much less prone to issues with ground offsets. The general recommendation is to wire to the ECU ground for these sensors.