Do you have to use low whistle fingering on a Low G whistle?..
I’m sure that once again I’m the oddball, but I use half & half on a low G because it’s not quite large enough for piper fingering, and a bit too small for fingertips only… at least for me.
I use the fingertips of my left hand, and piper fingering with my right.
Of course the correct answer to your question is to do whatever feels most comfortable and natural to you. 
I’m with Raindog on this one. My answer is the same. Whatever works for you but I’m most comfortable when I use the finger tips of my left hand and piper fingering with my right.
I think that most guys could comfortably use fingertips down to an F. G whistles are a little too short to use piper fingerings. -JP