I have discussed here previously the importance of proper voicing for woodwind instruments. In a nutshell, voicing is the configuration of the oral cavity, manipulated by moving the back of the tongue.
There seems to be some debate about voicing: is it something static, or something that changes from note to note? I find that the answer is, sort of, both.
For beginning woodwind players, tone, intonation, and response (virtually every aspect of tone production) can be improved by habituating a single, stable, “correct” voicing. When this is accomplished, an ideal woodwind instrument, which of course does not exist, would play perfectly in tune, with perfectly consistent tone from note to note, and with perfectly even and reliable response.
An instrument that is merely good will do these things well, but imperfectly. A more advanced musician can use small and temporary voicing adjustments to improve individual notes by altering their pitch, tone, or responsiveness. Doing this requires a “stable” voicing as a stepping-off point, fine control of the mechanics of voicing, and an ear trained to hear notes that are out of tune, uncharacteristic in tone, or problematic response-wise.
So, in general, when working with beginning students or others with significant tone production issues, the goal is to work toward a stable voicing that stays the same from note to note, but with more advanced students the goal is to learn to adjust the voicing ever so slightly to improve each note as needed.