A very clever person in this forum, fiqured out the difference between wiring the voltmeter to the acc terminals and through a relay to the battery, is 0.02volts.
That would only be true for a specific load already on the aux terminal circuit and then only for a load that was constant, and not fluctuating.
The actual measurement is not important if the load is constant. All you really need to know is what is the difference between the battery terminal voltage and the aux terminal voltage (whatever that may be). That can be determined one time by measuring at both terminals while the bike is running.
Once you know you the difference, every other measurement read at the aux terminals is relative to the battery voltage (again assuming a constant load on the aux terminal circuit).
This isn't a science lab. You don't need to measure absolute voltages and you don't need to worry about 1/10th of a volt. The meters that offer this are just providing a gimmick and add unnecessary interpretive confusion to the rider. Battery voltages monitored in the nearest 1/2 volt are plenty to determine charge system electrical health.
What is more important is to learn the pattern of charge cycles in when the system is healthy and new so you know when that patter changes.
Even SmartLight can provide that and it does it with a single sample reference point.
Does it hurt to add the relay? Other than cost and hassle, no. Does it hurt to go direct to the aux terminals? Not if the load on the aux power circuits is constant (from the other items attached to it) and you know what the difference between the battery terminal voltage is vs the aux terminal voltage measured under identical conditions. And that would only matter under extreme differences between them. If that were the case some investigation would probably be needed to identify why there was such a difference.
The voltage measurement point for the sake of this discussion is a tempest in a teapot, IMO.