This is a bit like resting breathing rate, the lower the better, but that's controlled completely unconsciously. Your lifestyle will influence it a lot though (exercise, relaxation, diet)
And this is going to sound weird, but I've taken it even further by buying some baby chew toys off Amazon and chewing on them for maybe an hour a day. The benefits I've seen since starting about six months ago are 1) less teeth stress/pain at night and in the morning, 2) less nasal pressure/stress (pretty sure I have more breathing capacity through my nose now), and 3) I feel my face has widened ever so slightly but that may be the jaw positioning alone.
Was there any reason to use toys instead of just chewing-gum?
Not really, except it lasts longer and I dislike the taste of chewing gum. Mastic gum works well, too.
https://www.youtube.com/user/sarahkh12
Tongue posture is a real thing, which was news to me! The therapy fixed my breathing, it was miraculous! An improper tongue position can inhibit nose breathing. If the tongue doesn't seal the airway, you will have a turbulent and inefficient breath --- 10,000 times a day.
I think another possible issue is that some people lock up their jaw without necessarily putting the teeth together. At least I do this if I don't pay attention. By "lock up" I mean that the jaw is simultaneosly pulled down by muscles and pulled up by the masseter. Then at night, the downward pulling muscles relax, and the upward pulling muscles (masseter) wins, and the actual teeth grinding occurs.
Seemingly coincidental remedy, too. I don't remember reading anything about the proper resting position of the jaw. I just found that the one the helped the other. In fact, I was worried doing this with my tongue was a no-no, but figured it was better than the alternative.