We all use it for our low D whistles, and no one (I think) would use it for a high D. But on what whistle do you make the transition from regular fingering to piper’s grip? B-flat? or A? or what?
Generally, I play an A with piper’s fingering, and a B-flat with standard…
There is surely no hard and fast rule here. Play with whatever grip is comfortable. I personally cannot get to grips 
with the pipers grip on the larger whistles so I play reasonably successfully with the standard grip on all my whistles. Then again, I am blessed with hands like coal shovels so this is not a huge problem for me.
Jetboy
For me it depends on the actual whistle. Occassionally I’ll use pipers grip on a Bb whilst on another A I’ll use standard grip!!
I don’t use pipers grip at all.
Just whenever it feels comfortable. I might use it with the right hand and standard fingering with the left. I might use it on a given whistle one day but not the next. It might depend on how tired I am.
Once you get used to both grips, you might well feel equally comfortable with them. I do.
I began using pipers grip three years before I ever held my first real uilleann chanter. I began low whistling on a low D and actually moved up the scale as time wore on (kinda backwards according to some, but thats me!).
Even in the high D and Eb whsitles I play, I use a very flat fingering that which puts the tone holes at least mid way back on the first pad of each finger. I have never used fingertips to play anything as it always come off (and sounds to me when I hear others play this way) quite sloppy.
As others have mentioned here as well, it really does depend on the particular whistle rather than just the key. I also find myself making a gradual transition down the key range and pushing my fingers further up (moving the relative position of the tone holes further back along my fingers) as the key drops.
I’ve used full pipers grip on keys as high as Bb and A, and been able to use a more “standard” grip on whistles as low as Eb and F. Never found a low D that you could play well without it. It’s actually a very relaxed grip and I would highly encourage anyone to work on it if they still struggle. You’ll be amazed at how much more speed and accuracy you can achieve once it becomes a more natural hand posture.
Best
B~
I haven’t used it yet. I occasionally will try the low D but lose patience with it. I like the agility of the higher whistles, they’re just more fun to me!
I use Piper Grip at the pub, when the flamin’ piper grabs me Guiness!
Grrrrrrr!
serpent
Low G & low F depend on whistle and mood… I can go either way on them (and don’t play either key much). Below low F: piper’s grip.
I was switching between guitar and whistle in a session this weekend, and my right hand was starting to get numb, so I couldn’t cover my SweeTone’s lower 3 holes reliably, so I used a piper’s grip w/ my right hand. I then switched to a cylindrical whistle (Susato, I think) and realized that this was really a good grip for me for cylindrical whistles of all types. I still use a fingertip grip on my high conical whistles, though.
I once watched a piper who had recently picked up the whistle, and he always used a piper’s grip (go figure), and seemed to have a lot of economy in his finger movements… so maybe it’s not so bad for small whistles, either. 
I always use piper’s grip- regardless of the whistle key- I can’t imagine using fingertips. But then I was a piper first.
I use the Piper’s grip on me low whisltes and not on the high ones.
Everything but the high D.
Yeah, Jim, Me too. Now that I look at it All my high ones are Ds.