Personally, I definitely prefer the talking character to have their mouth open.
Yeah, In real life, while talking, they would close their mouth inbetween - but when we think back to a speech of a movie with a talking scenne, we wouldn´t associate somebody saying something with a closed moth. How the the mouth opens is definitely a big part of human expression, too.
If I want to show a mouth closed emotion, I´ll jus not draw the talking character but the one reacting to it in a certain way.
I think is also a matter of your layout choices for specific pages. (if the speech bubble is way above the talking character, it then makes sense to me drawing them after they´ve finished talking), or if you even choose to include the characters mouth in the shot you draw.
Lol It´s not rocket science though, it´s about what people prefer to do. In the end it matters how yoru readers receive it and if they are confused by closed / constantly open mouths than that says something.