Jim,
for show shots, some dogs are supposed to have their tongues out (GSDs, Sammys) and for some it is an acceptable alternative (mostly the working breeds, Newfies, Saints, Rotties and Herding: Corgis egs.). For all the others, mouths closed if possible.

Now, if you're shooting outdoors in 95 degree weather, you're not going to get many closed mouths even with lemon water spray. It ain't gonna happen.

With pets or dogs for pet portraits, getting a closed mouth is even more difficult. You'll notice when I pose Rowdy outdoors, I have no problem with him "smiling" to show off that black mouth. He has good pigment and even breeders can appreciate that. But in the studio, I always try to get some with the mouth closed. I personally like a smiling dog and I find that when they smile the eyes twinkle a bit. But, as Peggy Sue says, it all comes down to what the owner wants.

With that in mind, it's a good idea to figure out how to get a dog to close their mouth when you're shooting them. There are all kinds of physical tricks but it comes down to what will work for each individual dog.

Jim