As an editor, I always look at the manuscript first. Always. My first approach to a novel is as a reader, and I want to be exposed to your writing without having been “prepped” for who’s who and what’s happening by the synopsis.
As a reader and as an editor, I want to see two things in the first few pages of the manuscript. First, a narrative hook that urges me to read more. It should intrigue me right away, rather than make me wait several pages to learn there is indeed an interesting character in an interesting situation. It should present action, dialogue and/or emotion—or at least challenge the reader with a puzzling circumstance.
Second, I want to be able to identify the protagonist and the setting, and understand the story situation—again, without having been informed by the synopsis. I have received manuscripts in which much of what is happening during the first chapter or two is puzzling—unless you’ve read the synopsis. (Or, until you’ve read past Chapter 6.) This is because the writer assumes people will know what she’s talking about, or because she is too focused on the story to remember that the readers don’t know everything she knows.
This is not to downplay the importance of the synopsis. It serves several purposes. It gives an editor an idea of where you’re going with the story. That saves time, of course, and lets the editor know that you are not writing a cliché (“…and it was all a dream!”) and not rewriting a favorite novel or film. It also shows how well you’ve organized your story—especially important if it is an episodic tale, switches viewpoints, or is not told in time-linear fashion. A good synopsis is in some ways a map of your novel that shows in brief the high and low points as well as the major elements of the plot. With this map, an editor can see where your story needs a little redirection, whether a character or event needs to be emphasized, and so forth.
To sum up: write your novel in such a way that the reader doesn’t have to read the synopsis to understand the story. And don’t put anything in your synopsis that’s not in the manuscript.