Posted on the Flute Forum but relevant to whistlers: Second octave D fingering - why vent?
Playing with no vent is the only “bad habit” I hot in early days of my playing that is still with me. I’m not going to do anything with it, actually, as it does not bring any problems.
Oh, OK, there is one. Some whistles (e.g. Every Whistle) are out of tune when 2nd D is not vented. I just avoid these 
Venting just makes the note sound clearer on some whistles. Some whistles, you don’t have to, but it’s still a good habit to be in.
As with flute, there are times I find it handy to be able to easily use both a vented or an unvented d.
Learn both; it can’t possibly hurt you.
–James