The application of Plumber's freezing spray might not work on whistles that have not already been adjusted, since it's a glue that holds the mouthpiece on, not just compression. Once the glue has been removed, adjustments should be easily made. The biggest problem is that the "wiggle-room" is so slight that it may not make any difference.

I simply do not bother. I typically do not notice a problem with my whistles being out of tune with the other instruments. I recently played several tunes with a tuba player and had no problem with being in tune. When it comes to guitars and other string instruments, in Florida the humidity and heat and A/C, etc. are always causing musicians to be rather flexible with their tuning issues. Some times, it's my whistle that someone will tune to, as they know that it never really varies.