It's not a bad idea to teach your dog to accept a muzzle. Why? Because if your dog needs a muzzle to keep him from biting someone, you are protecting your dog, not just a person. Think of it this way: you don't want your dog practicing biting.
But too often, people misuse muzzles or choose the wrong muzzle.
There are really two basic muzzles to choose from.
One that is like a nylon sleeve that goes over your dog's mouth and basically keeps his mouth shut. This muzzle is for use for momentary things like being at the vet, or if your dog doesn't like his nails done, etc. It should be on the dog no longer than 1 to 5 minutes or so.
When a dog is wearing this muzzle it does affect his breathing and he certainly cannot pant which is essential to a dog's respiratory system. Panting is also part of being stressed which is brought on by any procedure that your dog doesn't want done.
If you have a dog that requires a muzzle for any length of time, the one to use is called a basket muzzle. Your dog's face and neck and muzzle should be properly measured for this. Here is a link to my favorite muzzle. It is called the Baskerville muzzle.
This type of muzzle is what you want to use for muzzling your dog for any length of time as the dog can then pant and drink water. You can also treat him through it if you are trying to desensitize him to something.
Muzzles can be a very wise and safe choice for several preventive things. Just keep in mind that when your dog is wearing a muzzle you have taken away his way of defending himself. In other words, using a muzzle at the dog park is not a good plan. If your dog cannot be with other dogs without being muzzled, he shouldn't be there. It's not safe for him.
I hope this helps you sort through the vast amounts of information about muzzles and which kind to use.