When to use ellipses in quotes:

Use them to leave out a non-essential word or phrase. (“Marco was really, terribly disappointed.” “Marco was… disappointed.”)

When NOT to use ellipses in quotes:

At the beginning and the end. (Your reader assumes that the author wrote something on either side of the quote. There’s no need to indicate that you left off half of the book.)

When leaving out something changes the meaning.  (As in, changing Nixon’s famous “I am not a crook” to “I am… a crook.”)

When you leave out such a big chunk that the speaker has changed. (“I love you… I’m sorry, but I don’t love you back.” This would leave your readers saying “Huh?!”)