I'm currently using a collar for my lab. Am I being a bad parent for not at least trying to use a harness? My wife has been complaining at me for a while on this, so any feedback I can reassure her with would be helpful

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If your dog doesn't pull, a collar is great. Some prefer it if they use leash cues (
gentle pressure in various combinations to convey different commands, like heel or sit) because the dog feels it more, though I personally have found that my dog feels it on the harness just fine. If your dog pulls, though, having a collar on could cause throat problems, so you should use a harness, at least until you have finished training. Also, if you use a long-line, you'll need to use a harness to prevent severe injury if the dog runs to the end of the lead. It's a combo of personal preference and practical application.
And, as
@Dave T said, a harness often encourages the dog's natural pulling instinct, so it isn't the best idea unless you are also doing loose leash training- if you're training, it's fine.