Trade off about lifting the corners of the mouth.

This topic is very important.
Maybe, you’ve heard that vocal coaches often say, “Lift the corners of the mouth. Sing like you are smiling!”
I’ll explain some positive and negative points about lifting the corners of the mouth, when singing.
Good point:
When you lift or extend the corners of the mouth, the space of oral cavity and pharyngeal cavity enlarge to maximum capacity.
The space is really important to make harmonic sounds or echoes, just like a music instrumental. Therefore, this stretching causes the voice to increase the potential to make the voice better.
Bad point:
When you lift the corners of the mouth, the orbicularis oris muscle is stretched and the front teeth either stick to the lips, or the front teeth are bare. When the orbicularis oris muscle does not move it obstructs the movement of the lips, which tunes the voice, and becomes harder to produce good sound.
Please use the way that is the most comfortable and suitable for you, after you understand what the exact trade off is.