Al, I stop the sound because silence is every bit as much a part of music as airflow and sustained tones. Many times a Tahhhhhhhh just doesn't cut it. Taht sounds more like the string players pizzicato. I start with articulation VERY early with my students. They get enough breathing, long tones and slurs for the breath support and flow. If I think about much of what is said in school bands/orchestras about the brass players, what comes to mind is of course timing, but VERY often too massive and not articulate enough. I think concentrating on flow needs to be tempered with a feel for the situation. There is nothing that holds a beginner back from imitating speech, other instruments or sounds.