When to blow the whistle?

I am referencing the end of the Indiana-Illinois game. Illinois player goes up for a layup with 2.5 seconds left, Indiana player swats it horizontally out of bounds. I noticed the official blew his whistle (thus stopping the clock with .9sec) before the ball hit the ground.

It didn't matter in this game. I slowed down the tape and estimate that the ball hit the ground with .7 seconds left, which would have still been enough to hit the game-winning layup.

My question: if the ball is clearly headed out of bounds with no chance of a player returning it inbounds, should you whistle right away or wait until the ball is dead from hitting OOB?