I see and hear people all the time use bending exercises, but not for centering the sound. Perhaps they think it is a kind of lip gymnastic
Two weeks ago, I asked Bobby Shew (he had i Clinic here in Norway) about bending. He said that he did not use bending, but he had developed a type of vibrato exercise. It all started a long time ago when he tried to play like Clifford Brown. When he did the Brown type of vibrato he discovered that the sound "opened up".
Some years ago I asked Hardenberger:
What do you do when you bend?
He replied:
What happens is, that you make a small vocalising (sings halftone interval O-AA-O) – just a small change in the throat. You make no extra push with the air. If you start to push with air, you introduce a fight against the trumpet instead of letting the trumpet “walk” with you and let your mind control the trumpet. I use to tell a little metaphor: When I was a little kid we had a very big dog, a St. Bernhard, and when we were out for a walk, he was out walking me. In my relation to the trumpet I try to be different: I decide over the trumpet and not vice versa.
More here:
http://abel.hive.no/trumpet/hardenberger/
Ole