For the editors I hire and pay, I feel like I get the final say. I am open to anything they suggest, but I don't always listen. If it's the brass-tacks of grammar, punctuation, etc, I'm pretty much clicking "accept change." And most of the time, when I don't listen, it is when it comes down to something that could go either way and our differing of opinion on how a character should feel/react. Ultimately, I feel like I *need* to have final say when it comes to that -- it's not a lack of respect for my editor -- but when it comes to my characters, I think I know them better/have lived with them longer.
For example: I had a line in a book about a character who has stayed in an abusive relationship. Her parents were divorced and, right or wrong, she chose to stay in her marriage because the divorce of her parents was hard on her and she didn't want to put her daughter through that. My editor felt like that was misnomer because lots of children come from divorced households and are totally fine, well adjusted, very much loved human beings. Of course they are. And my saying otherwise in my book wasn't painting an entire subset of people with a broad brush, nor was I saying I believe that...it was about one character feeling one way about her life. That's all. No more, no less. Ultimately it came down to staying true to my character in that one instance...and later on down the MS, I did make a change she suggested.
Like with any relationship, working with an editor is a collaborative effort. She sees things I don't, I understand things she doesn't. In a great relationship, you balance each other out and the end result is a fantastic book.
Now, with my agent. Totally different ballgame. I can't necessarily overrule her with ease and grace.