What’s going on in your mind is far more important than specific mouth shapes.
I find that if everyone is very clear about exactly what vowel the group should be thinking, then a completely relaxed, neutral mouth shape will allow the words to be crystal clear, and the sound well blended.
The synchronization will also be better because jaw hinges will not be moving different distances, and different speeds.
This technique works best when the chorus already understands the concept of target vowels, and diphthong/resolution vowels, and the importance of a relaxed tongue during the singing of the longer, initial (target) vowel.
Light = lah________eat
Day = Deh_________ee
Rose = Ro_________ooze
A high soft palate will also allow more resonance – making it unnecessary to open the mouth wide.
Eventually, when the group gets a sense of how little movement is necessary, I do allow them to open up more in the forte sections at the climax of the song.
When we train our minds to focus on the clarity of the vowel, the group’s sound is rich, resonant, blended and synchronized.