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It depends on the age of the dog and where the dog is in training. If he has the fundamentals down and is working on proofing (performing learned behaviors in spite of distraction) then an active place like a busy shopping center might be a good place to find those distractions to practice ignoring.
If it's a pup being socialized and he's confident and outgoing, a busy shopping center gives ample opportunity to meet new people.
The key is to do it at the right points in the dog's development or training. Overfacing a dog by putting them into a situation too stressful or too distracting for them to process will only slow or damage socialization or training.