I dont play whistle but from a brass players stand point that has messed with recorder teaching youngsters, I teach finger tips. The reason for me is the speed and it is more a straight up and down motion, when playing with the flat part of the finger you unconciouly push down and in just because the physics involved. so I teach Finger tips so that when they move to a brass or woodwind intsrument they have good habits and not bad ones.

But that is just froma tuba players point of view.