I guess the decision is made at the UPT step before the fighter boys go off to the T-38 but a friend of mine told them all along he'd rather go tanker than fighter/bomber and he got his first choice of KC-135 at his drop. The only "disappointment" he had was that he was off to Europe rather than staying in the US.
He said from day one of training you can tell who'll be the fighter guys, who'll go bomber and who'll go tanker/transport yet they still do tests and interviews to decide, the US system is a bit different to RAF's from what I read.