If you look at the book “Dossan of Heather” you’ll find plenty of tounging and if that music is not ITM I sure can’t imagine what is…
Of course you can tongue on the whistle. It’s a legit part of your repertoire and I use it quite a bit, along with my fingerings. I find it helps punctuate phrases and can help things “pop” in a unique way. Come sit next to me next time… 
Anytime I receive unsolicited advice on my playing I diplomatically point out that my playing is infinitely better than finding a huuuuge key scratch on the side of one’s car.
Hmm.. Interesting..
Well, I play pipes and I also tongue on the whistle. I like to think that I’m emulating the pipes by tongueing [Is that spelled correctly…?] because it’s similar to the tight style. Although I’m kind of both. Well, anywho, that’s how I do it.
How was Willie Week?
J.
I’ll tell you about Willie Week if “Murphystout” tells us how he got on with Kevin Henry !!
I’ll pass that request on to the man…
J.