Is the only reason to teach sign language to a dog if you think he/she is going deaf? No! When you use both verbal and visual commands, and teach the dog to resond to either one, then when you do use them together, it actually sends a stronger message, and the dog is more likely to respond. I teach all of my dogs these commands with both verbal and visual cues:
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When I teach hearing dogs these commands, I teach them using verbal and visual cues together, then once they fully understand that command, I start to teach them to do the command using either verbal OR visual cues. Do this by using both a couple times, then the third time drop out either the verbal or visual, keeping everything else exactly the same, if possible. This is demonstrated below.

You train a deaf dog to follow the treats and they pop in their mouths when they do a certain thing. This behavior will increase in frequency when the dog is rewarded for doing it. You then do the same thing as explained for hearing dogs- drop out one thing, keeping everything else as similar as possible. So if you are teaching down, first teach it by having the dog follow the treat onto the floor. Do this a few times in each training session and then end it. Once the dog has gotten a pretty good handle on how to get that treat, then increase the speed in which your hand goes toward the ground, so that he/she does not actually get to smell what is in your hand, they just know that there is a treat in it, so they follow it. your regular way of having the dog follow the treat down to the floor, a couple of time, treating the dog each time. Then keep the treat in your signaling hand, but then actually reward the dog with your other hand (the hand that is supposed to be the treat hand). Then take the treat out of your signaling hand and try it that way.