pm_me_your_innie
I've been doing this and it has been helpful.
The main reason I tried it was that my snoring was bothering my wife and it only happened when I was breathing through my mouth. I also had my dental hygienist comment that I must be a "mouth breather" when I'm sleeping, despite not being one while I'm awake.
I use a Breath Right strip on my nose, and the tape I use only keeps my mouth from opening involuntary. I could open my mouth easily if I tried, so I'm not really worried about suffocating or anything like that.
It felt a bit weird at first but it has definitely curtailed the snoring, so my wife likes it. Maybe it's not for everyone, and some of the stories people tell about it seem pretty woo-woo, but it does what I need it to do with little trouble.
This prompt generated a ton of really good costumes, this image just happened to be closest to what I was aiming for with the least problems.