It all depends, though, on what the writer wants. To me, having my grammar corrected -- politely, of course -- IS positive feedback. I try to deliver it to others the same way, and with the explicit caveat of "This is what I suggest, but do what you feel is right." There have been times I've actually suggested using non-technically-correct grammar, because it fit the style, the character, or simply flowed better. But variance from "correct" should be an artistic expression, not an act of ignorance.

And that's really what it comes down to. Authors own the work, and need to keep it true to themselves, but they should be aware of what they're doing and why.