Depends on the harness and why you need one really!
Some harness are designed for dogs who pull, such as when walking. Others are designed for dogs to pull, such as a sled.
I use a harness on my dog. Because of her high prey drive. She does perfect loose leash walking after a couple weeks of training with her on it. However, she wears the harness because if she see's a cat, and i cannot redirect her in time or if its too close to it. She will bolt at it. And i do not like her doing that, because it can hurt her throat and cause damage. Thus, for now, she wears a harness everywhere. And i take it off at night or when she is crated.