Curious, what do you consider and “odd” key?
Good question though. But really, while we can all tell you what our experience has been only you have the right answer because it is entirely dependent on your church, their music preferences, their music director and the flexibility of the other musicians.
My simple answer is that you will need a whistle in every key. A, B, Bb, C, D, Eb, E, F,and G, will surely come up. In some cases it will be best to have high and low versions of some keys like Eb, E, D and C. The vocal range of particular singers may push you to need the other in-between keys.
I played whistles (and many other instruments) in church for many years as a worship leader (I’ve since retired from all that)- playing mostly contemporary Christian stuff but also more traditional stuff on occasion. I played a wide variety of whistles and flutes depending on what was needed. I have a set of ~24 Burke whistles in most keys and materials that I used most weeks. I used a large set of Susato Kildares for key coverage before I moved to the Burkes. I found it helpful to stick to one type or make of whistle so that I didn’t have to stop and think about what a whistle in a particular key required when I picked it up. Burkes work exceptionally well in that regard I also make my own whistles and they would fill in the really odd keys when called for (like C# or F#). I also used Gens, Copelands, Thin Weasels, Sindts and others when I could. It really depends on the setting, how much volume is needed, which tone works best with the other instrumentation, how formal or informal the service was, etc.
My suggestion is to build your whistles case for key coverage by starting off with a full set of Generations as benhall.1 suggests. They do not need to be tweaked as has been said. That will give you much of what you need. Remember that each whistle plays easily in two keys. So a D whistles plays in D and G; a C whistle plays in C and F; an Eb plays in Eb and Ab; Bb in Bb and Eb and so on. Then add a B whistle (keys of B and E) when those keys are called for. Same for the A whistle, low G whistle, low F whistle and so on. Eventually you will have a whistle in each key you need.
Yes, I like the Syns too, but the single head solution wouldn’t have worked best for me. Maybe you could get a set from Erle with two heads.
I found it helpful to have a separate whistle in each key rather than one head. The reason being that it is not unusual to modulate up a full step in a praise song. Swapping heads at the right time doesn’t usually work. Picking up the whistle key in the new key is more doable. Something like “Days of Elijah” is one example for your consideration.
Also don’t be afraid to inquire about a change of key one step up or down in a piece. Guitars have capos. electronic keys have “transpose/pitch shift” settings, drummers don’t care. Singers can usually go up or down one step. Brass players might protest.
Have fun. Make a joyful noise. Check out the Praise Whistlers too. http://www.praisewhistlers.org/
Feadoggie
Actually the B whistle is rather common for Praise music, at least in my experience, A fair amount of guitar based music is played in the key of E which can be played on the B whistle. The usefulness of the B whistle actually surprised me at first. The B whistle just sits in a nice range sometimes where the high E is perhaps too chirpy and the low E seems to get lost a bit. So really what we might call odd keys can vary. The F# whistle is rare to me. I’ve made one and used it once, but yes, it was for a praise song.