Singing with a relaxed jaw is good.
But, Guys and Gals, in can be disruptive to one's singing.
Think about it like learnnig to ride a bike. When you're very young, you start on a trike. It's safe, you aren't going to fall over but, as you use bigger tricycles, you find that cornering is restricted.
Later you go to a bicycle, but you fit stabilisers, usually fairly close to the ground. So sometimes you're on two wheels, sometimes three. Gradually you raise the stabilisers, and finally you ditch them altogether, and you're away in total freedom.
I asked a Laryngologist why it is that relaxing the jaw is so disruptive to singing. He replied that the "muscles of the jaw are closely connected with the muscles of the soft palette and the middle ear."
I my experience, I would add to that list the muscles bringing the vocal chords together ("the vocal kiss") and the muscles tensioning the chords themselves (ie those making the pitch go up and down).
When you let the jaw go loose, you clearly let some muscles relax. But the voical instrument still needs muscles around it for stability and control, and so you have to sense where those muscles are, and you have to re-learn how to use them, and if necessary, develop them.
I have found that trying various actions - starting a vowel, singing on a "Z" or "V", raising lowering pitch, raising the soft palette - with both tight and loose jaw is a very useful exercise. You'll start to notice how you managed these actions with the support of your tight jaw, then you have to see how you do it with a loose jaw. You have to be aware of these quite small changes, and then add the support back in, otherwise the voice can just wobble around and be very unreliable. I felt I had to completely re-learn how to sing in the middle of the voice, before it was worth exploring the upper register.
So, my advice is, don't try and transition between tricycle and bicycle too quickly, or you night fall off.