Yes, it's true that beginners tend to keep their corners shut and can't then breath properly through the mouth, but then when are we going to teach them corner breathing through the mouth? When they are about to audition to attend Juilliard or other conservatory? At the most, we'll be lucky if they've learned it well in 7 years viz just 364 hours of weekly sessions. Though I won't dispute the effect of nasal breathing to set up a high register "tessitura entrance" and hold the embouchure in place while performing it, I've no expectation of a beginner in Book 1, of being capable of doing likewise. However, now in their 8th week and about to finish Book 1, I've observed all 4 of the beginning students I'm presently tutoring breathing through their nose naturally at times as needed and I'm not challenging such while I put forth the corner breathing technique, which they are only so so doing presently. Too, my focus centers on tonal sound production rather than speed / tempo at this stage. Still, I know every student I now have expects to play in the high school band and I must do all I can for them to achieve such. Based on my own experience, they best then have corner breathing ability of at least 80% or higher as the competition now becomes very keen. I still don't know how I was able to play 5 years in high school band, yet only as a regular trumpeter for 3 years and even then from time to time playing other brass instruments as needed.