There really is no wrong way to hold a mouthpiece when buzzing, but there is a way to hold it that can eliminate a common mistake made by students.
Many students figure out pretty quickly that it is easier to "hit the notes" on a mouthpiece when you jam the mouthpiece into your face. This especially goes for higher notes.
Of course this is something that needs to be discouraged since it leads to both fatigue and a terrible sound. I have finally noticed a correlation between mouthpiece pressure and how it is being held (especially for the low brass instruments.)
When holding a mouthpiece with the entire hand, it is difficult not to apply pressure when buzzing.
But when holding a mouthpiece with only two or three fingers, it is difficult to apply pressure when buzzing.
By simply having students hold the larger mouthpieces with only three fingers at most you can avoid the issue of excess mouthpiece pressure without evening saying the words.