It's also considered good manners to edit away the parts of a quoted post that aren't related to your reply. So if I post about my Christmas dinner, the clever thing our daughter said, and my edits, and your reply is only about my edits, people joining the conversation late don't have to read about that dinner and daughter.
And it's super good manners to insert <snip> or something else to indicated text is deliberately removed when it's not obvious. Usually it is, but not always.
Maryn, who liked the Miss Manners books