Personally, I feel if you don’t make fair, well-timed corrections part of your training you’re setting yourself up to go into the ring with a dog who will probably do as he pleases once he realizes no tangible rewards or additional handler help are coming. Depending on the alignment of the planets, this may result in a qualifying performance or it may not. (Exercised Finished - Are Corrections Really Necessary?)
What IS a correction?
...• A correction only needs to be strong enough to get your point across; if it doesn’t make an impression, you’re just nagging your dog and that’s not going to fix anything.
• A correction addresses the problem at the point where the error occurred (for example: at the point of pickup on a retrieve or during a slow response to signals)
• It is better to make 1 effective correction than 6 naggy ones.... (Exercised Finished - Corrections Part I)
Punishment techniques should not be taught to novices. It is an advanced technique. (Bob Bailey, The Fundamentals of Animal Training, paraphrased from memory)In the example of the ubiquitous of the leash pop, the timing, magnitude, and attitude of application are of paramount importance.
It must be strong enough to make an impression (too low a level will require more frequent application and risks habituation - the "punishment callus") but not so strong as to overwhelm and shut down the dog. It must be strong enough for the dog to wish to avoid it in the future, but no so strong as to overwhelm the dog's ability to think through how to do so.
Timing of a leash pop is even more important than timing of a click. Mistimed clicks lead to frustration but generally if you're doling out good enough goodies you can keep the dog with you, mistimed pops lead to a frustrated dog that is more likely to say "Screw you! I quit!" than work through the frustration of handler error to figure out what IS wanted.
Attitude of application is something that I have heard varying reports on. Some say corrections should be impersonal, the dog should think they come from the environment. Other say the dog should know corrections are issued by the handler. Most agree that you shouldn't feel anger towards the dog, that the dog is a "bad dog" and that "you'll show him, he'll never do THAT again!" But there is a well understood connection between actions and emotions. Looking for things to correct puts you in a different mindset than looking for things to reward. It sets you up for a more confrontational attitude with the dog in training.
Applying fair and effective corrections that end unwanted behavior and decrease its frequency in the future is a mechanical skill. Novices have CRAPPY mechanical skills. Philosophically, I do not have a problem with skilled trainers applying fair corrections to their dogs to answer questions about an exercise they have performed correctly hundreds of times before. "Do I have to when there is a fox in a box? Yes, you really actually have to." In my training I prefer to avoid that if I can, but when I am Queen of the Universe I would allow other people to do so. I do have a problem with pet owners training their first dog popping their dog because he didn't auto sit, because they're going to do a piss poor job of it and confuse the heck out of their dog.
When they can effectively handle clicker, treats, leash, dog, and a prop, maybe they are read to start learning about leash pops. But at that point, they probably don't need them.
Safety of the dog is paramount. In immediately stopping, dangerous, bad behavior - not just unwanted or naughty, but outright BAD - I am not above using punishment to stop behavior in the moment. If my dog is trying to eat an entire dark chocolate Easter bunny, you bet your ass I am going to yell and fling him away by the scruff. But I do not consider it especially effective in preventing him from trying it again next Easter.
Corrections are reactive. Positive reinforcement is proactive. Be as proactive as you can, but I do not think it is a dog training sin to have reactions in your bag of tricks.