It occured me today that all my whistles are in the key of D. Are there any other keys it would be good to have or is that kind of unnecessary (surprizingly, I have yet to fall prey to the need to own every whistle I see)?
I personally play about 99% Irish music. My whistle needs are usually met with a D, C and A whistle.
Hope that helps.
Mike
It depends on what you play and what you like. If you play with a vocalist you need more keys. If you play with other instruments you may need more keys and they should be tunable. I play different keys for specific tunes because I like the way they sound in a different key. Its what you feel you need.
Whistles in F are sweeeet. Low F’s more specifically
I play mostly in D, but really appreciate playing alone on Bflat and G whistles.
Philo
I play my C whistle the most. Its a very sweet sound.
As noted above, D whistles cover most tunes but in addition to a C and an A (good for tunes in D that reach down below low D), a high E is best for most tunes in A.
To add to the above post… I agree with the keys recommended here. Also a whistle in high E (NOT Eb) is good because sooner or later you’re gonna run into some guitar player or group that plays contemporary tunes and many of those are in E.
Dave
the G is helpful if you are playing with people playing or singing
in C. The tonic (three fingers down) is C, so you can
use what would be the ‘G fingering’ on a D whistle to play in C.
Similarly the key of A gets you an alternative fingering for D.
Bb and F are good keys for playing while chicks are singing. 
Doc
I agree with Mr. Kiley in that it depends on what you play. I play a lot of non-ITM so having a complete set is versatile for playing along with any music.
Also having other keys helps break up the monotony of playing in D/G, and A. A B-flat or F is a nice change of pace.
If I only had 3 whistle keys to chose from, I would probably go with a D, a C and an E-flat (or a Dixon C/D tunable which plays all 3!).
No, you looney tune, not THOSE sort of chicks!
I use a Bb or Eb for jazz tunes, and mezzo A and G for accompaniment in folk stuff. Low E is nice for tunes in A, but I use flutes for most low stuff. I never seem to use C whistles, but that’s just me…
True WHOA dictates that there should be no less then one of each. Then there is the question of how to find the one you want to play, so you will need a few extras. D, C, and Bb are a good start, but the Eb is fun to play sometimes…Then… Ya, one of each to start with.
It depends of course first of all in what key you want to play your music. And if you have a singer, it also depends on the key he sings in best. D is a much used key. But you should play a key that suits best to the music you play, and that can be any key.