“Most professional writers appreciate being edited for consistency and clarity; they want an editor to go over their copy with a fresh eye, to spot errors, to point out gaps in logic or sections that need cutting, to suggest where style can be improved – but they want this to be done with respect and tact. … Copy editors should be grammar coaches, not grammar police, and the final stage of editing the manuscript should feel like a collaboration, not an inquisition or a day of judgment. Some copy editors come across as school marms, with rigid sets of rules and an urgent need to rap knuckles every time they are broken.” (Copyright (c) Pat McNees [http://www.writersandeditors.com])
As I told a client recently, I use the Chicago Manual of Style and Merriam-Webster’s Collegiate Dictionary. I prefer not using “their” and “them” as singular pronouns. But I know this is gaining wider acceptance. So I simply pointed out the first few instances where he’d used this style and wrote, “OK?” in the margin notes. Since I was giving him several sections at a time, after he saw my initial comments he emailed me and said he preferred that style–he said it seemed more natural to him. So I didn’t mark any further instances of this usage.
In another instance, he asked me about my changing all caps (used for emphasis and/or shouting) to italics. I suggested he not use caps as much, but explained that since this wasn’t a hard and fast rule, it would be his call.
For the most part, unless it means a significant change, I don’t think that it is important to explain my grammar or style corrections to an author. If he or she were really curious he or she could check my reference books or could ask me (as he did).
For another book, the author had asked me to do a heavy edit. At one spot I questioned whether it would make sense for a certain character to be doing something, based on what had happened up to that point. This gets trickier, when you get away from grammar and style and delve into the actual characters and plot. This is the writer’s baby. I’m not there to rewrite the book. I’m there to help him or her make it read smoothly and not let mistakes jar the audience out of the story.
It is the writer’s call–even in spelling a word wrong or using a phrase incorrectly. But he or she relies on an editor to tactfully point out places where improvement is needed, or simply recommended.