I find myself letting a lot of "grammatical errors" go, especially in fiction, so long as the meaning is clear and the sentence doesn't cause me to drop out of the story. I won't force an unnatural sounding phrasing simply to avoid a dangling preposition or split infinitive. Naturally, if I think 'correct' usage will improve the piece, I will suggest it to the author; but if the author has adopted some common usage, I'll go with that because it echoes the reality of the scene the author is referencing and what the audience expects. Forcing "correct" usage in a context where the reader anticipates some common idiom is more likely to drop them out of the story, and so would do a disservice to the text.
I think this approach reflects a less intrusive editorial style than many other editors adopt. I am inclined to give priority to the author's voice, and so tamper only when I believe there is a problem. If I didn't trip over the sentence, I'm not about to rewrite it.
But I have seen other editors who pour over each word of each line seeking ways to improve them. Recently a top author showed me a couple of pages from one of her manuscripts that had a multitude of suggested revisions. I was astounded, because this was an author whose work I almost never find anything to revise. Upon close examination of the suggested edits, I had to concede that in almost every case, the language was subtlety improved. Indeed, I remarked that my novel would greatly benefit from this editor's suggestions, since it was obvious I could learn a lot from how she was tweaking the writing, making it tighter, sharper; both more economical and more effective. But. She was also fundamentally rewriting the author's work, and it really didn't need it. So the author's style is breezy and loose...that's actually part of charm of her canon. As an author with over 30 successful titles out there, reworking the novel to make it more "literary" was perhaps kind of missing the point.
So it is important to understand context. There are certainly many manuscripts that could benefit from a close line by line substantive edit. I could use that editor, for example, because as a relatively novice novelist (in spite of my other writing/editorial experience), I don't have an established following, I don't have a recognizable voice that shouldn't be tampered with, and I likely haven't yet reached my full potential (hope not, anyway!) So I would love to be able to have that editor edit me, both to fix the current manuscript, and to teach me how to write even better next time. Definitely a place for that style of intrusive editing. It's just not what I do.
Not that many of my clients would agree with that assessment for a moment. It is fairly routine for me to demand 400-600 major revisions on a novel manuscript, not counting copy editing of grammar and spelling and so on. But hopefully my suggestions are helping to make the manuscript clearer and more marketable, and not that I am being pedantic.