So once I know that my dog truly understands the command then I can give corrections using a variety of tools. These tools can include my voice, a training collar, or my hands. Now I really want you to know that like all tools, you must know how to use them correctly. If you do not know how to use any one of these tools, please talk to an experienced trainer and ask how they utilize them. So, let us say that I am working with my dog, and the dog ignored what I asked it to do, then I would use an appropriate correction in that situation. You must understand your dog to know what correction to use in each situation. A correction that is too hard can very quickly change the way the dog views your relationship. It is always important to maintain a dog that is excited about working rather than one feels it must do what you want because of possible reprisal. Now on the other hand a correction that is too soft will have no affect on the dog. While it does not hurt the relationship like a hard correction it can send the message to the dog that it can do what it wants when it wants over time.
There are other situations where a correction is needed even though the dog may not have a full comprehension of what you are asking of it. These situations can include gross bad behavior like biting your hands, mounting any person, or even urinating on someone. These are clear signs of dog dominance and should be treated quickly. A dog that believes it is the pack leader and is aggressive in nature can quickly become a headache and could even land you in court if it continues.
So the true answer to the question is you can use a correction once you know that your dog really does understand what you are asking it to do or it is such bad behavior that it must be addressed immediately. Remember to always have patience when training your dog. Think about how to motivate him before you bring him out and then make the training session into a party!