I just asked the question on
http://www.fibaamericas.com and here's what Fred Horgan, the FIBA rules interpreter, responded:
Although the FIBA Mechanics Manual does not require that the trail official give the "basket counts" signal on every shot for a goal, the reality is that whenever there is doubt on the part of scorerkeeper, player or fan, the person in doubt will look to the official to see if the goal counts. For that reason, it is a good habit to give the appropriate signal (basket counts; basket does not count) whenever the ball passes through or remains in the basket.
Hope that's useful to everyone!