I think it depends on the manner in which it is done. Firstly, I think "Fixed that for you" should only ever be used to make a joke or look at the situation in a lighthearted way. Far too arrogant to be used in a serious post.
Secondly, I think if someone has made a lengthy post and the 'editor' has changed a word or two here or there followed by a comment that says "I think this is a great idea! I would change these couple of words to make it more friendly/unfriendly/succinct/whatever, what do you think?" then that is reasonable to me. That is, it's taking someones idea and politely suggesting a way that you think it might be developed, and you are obviously open to be disagreed with.
Thirdly, if the majority of the post or the central idea/approach of the post has been drastically changed then I think it's really not on. If it is done to disagree with someone then I think it is rude - write your own post. You can quote them and then say "I disagree with this post for X reason. I think it would be better to say blah blah blah."
Having said that, it's never happened to me nor have I (to my recollection) done it to someone else so this is a relative 'outside perspective' fwiw.