I go back and forth. My last long term phone before my iPhone 6 was a Galaxy S3. I used that without a screen protector for most of the time I had it. There were a couple very minor screen blemishes when it came time to sell it, but I honestly don't thing it hurt the resale by any perceptible degree. I always take care of my electronics and gear but I definitely didn't baby it either.
Right now there is a screen protector on my 6. It's been on for about 5 months, and I happened to apply this one perfectly, so I'll leave it on until it starts peeling up or gets scratched.
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