TJ, what Dale was saying (I think) is that the tuner might cause you to rigidly play in tune so that the note is sounding correctly with respect to a tuner and not to those around you. If you are playing with electronic keyboards that are automatically A=440, then you're fine. But, what happens when you are playing with other horn players and they are in tune to each other, but not necessarily with A=440? I don't think that he was saying NOT to use the tuner, but to keep it in perspective. It is an excellent ear training tool and no one here is second guessing a teacher that we haven't met. He was just cautioning you to remain flexible.