Ah,found out where to ask this question.
Dear Whistlers:
Re. High D Whistle
What is your opinion on having all six holes covered when playing the D above bottom D on a D whistle especially when playing a fast piece ? Sometimes I do this as it’s easier, or perhaps it’s because I’m self taught and played a recorder before my venture into tin whistles. Should I make a determined effort to vent the first hole at all times ?
I understand it’s the same question no matter what key the whistle is in.
Whatever works best for you and sounds best is the way to go. Personally, I usually open the top hole since I find that this fingering sounds best to my ear on most whistles. I’ll sometimes keep them all covered in a fast passage too since that less satisfying note passes by too quickly to notice.
This topic was recently addressed here:
https://forums.chiffandfipple.com/t/how-important-is-lifting-the-first-finger-on-high-d-really/93341/1
Not for the first time. I try to always lift the top hole for the middle D.
ditto what brewerpaul said.
Thanks folks.What a wealth of information.
I followed through all the clips etc. and after listening to Matt Molloy I think I should quit - - not really.
On many/most high D whistles the sound of open v closed Middle D is pretty much the same, so it’s mostly an academic, rather than practical, question.
On the flute, and on many Low D whistles, there’s a distinct difference in sound: open Middle D sounds smooth, closed Middle D can be “honked” or “barked” and is essential for percussive rhythmic playing.
For example, play the second part of The Kesh Jig with all closed Middle Ds, “barking” all of them. It almost becomes a different tune, with a wonderful syncopated lift. The effect will be minimal on high whistles, moderate on most Low D whistles, and quite transforming on the flute. Of Low Ds I’ve played, the Burke comes the closest to being able to mimic the flute in this regard.
As I’ve mentioned before there’s an “old school” approach to fingering the Irish flute and whistle which involves raising as few fingers as necessary, which makes it much easier to play fast and clean. This is to be contrasted with the Boehm flute approach which involves putting down as few fingers as necessary. Closed Middle D is part of the former approach.
I was expecting Richard to make the point about ‘honking’ the middle D’ on the flute. In fairness, it should be made clear not all would think the sound produced that way a merit and using it all the time is perhaps not the best idea. Like most stylistic devices, choosing when to use an effect is much more effective than using it as the default position. So in that sense it’s probably best to adopt the open middle d’ and only close it when you want to go barking.
As whistles go, middle D’ will sound pretty much the same open and closed when played on its own. But on a lot, perhaps most, whistles it depends very much on how and from where the note is approached. When playing a tune like Fred Finn’s where a phrase like fdad bdaf | dfaf gfe occurs, the D’ will sound more effectively with less chance of it breaking if you vent the top hole. So in case like that the choice will be practical and less academic.
As whistles go, middle D’ will sound pretty much the same open and closed when played on its own. But on a lot, perhaps most, whistles it depends very much on how and from where the note is approached.
Glad you made the distinction flutes and whistles as I’m only familiar with playing whistle. The ref comes from the Flute Forum thread.
I learned to vent the middle D’ and later discovered that closed fingering also works, but, situationally while playing tunes. I’ve encountered seasoned players from both sides on the middle D’, open or closed. What works best is best. 
I almost always use the vented middle D but I play several pieces where there are a series of cuts on middle D and there’s a considerable difference in tone to the cut, a more controlled sound, if I close the first hole. I’m never really sure which to go for.
works for these two
https://www.youtube.com/watch?v=dLlxPt1yab8
and these two
https://www.youtube.com/watch?v=PexsKrghIr8
(look closely at the middle D’s in the second parts of the tunes)