I recently read that some guitarists who where called experts on Open-G tuning, Keith Richards was one mentioned, tuned the second string, which is the third in the chord, a little flat to tune it to the overtones rather than to the standard G-major chord.
A little theory must be insterte here. When a guitar string is ringing, it produces many tones. We find the first overtone when the string is divied in two, with the string vibrating as two halves rather than in the whole length. This is the note we get if we play an harmonic on the 12th fret (touching the string lightly at 12th fret when it is picked and released imediatly. This will isolate the first overtone.
The next overtone is the string divided in three – which give us a perfect fith above the first overtone (one octave + one fifth above the root). This is the harmonic on the 7th fret.
When the string is divided in four, we get the overtone one octave above the first overtone (two octaves above the root). This is the harmonic on the 5th fret. So far we can stay with the frets.
But the next overtone is one third above. But it is neither a major nor a minor third. It is slightly below the major third. The way we tune the guitar (and the way a piano is tuned) is a compromise called a tempered tuning or equal temperament. The distances between the notes are divided into equal half steps. But this is not in tune with the overtones. In a just or natural temperament you tune to the overtones. And this is what we can to in for instance Open G tuning.
The easiest way to in what can be labeled just Open-G is to find the fourth overtone, the harmonic slightly below the fourth fret on the 3rd string. Tune the 2nd string so that the harmonic on the 5th fret equals this fourth overtone on the third string. I have tried it with slide and it works very well.
As long as we do not move to far from the root, the just or natural temperament works great. But it will not work if we move far away from the root and into other keys. The equal tempered tuning was needed for more complex music with modulations to other keys, etc. You may also run into trouble if you play fretted chords that are not just barre chords, as you change the relations between the strings. (You cannot play a B on third string, fourth fret, as it will crash with the slightly detuned B on the second tring). And if the rest of the band is playing in equal temperament, you may run into trouble. But try it out.