Writing Tips (XXI) Internal Dialogue: Italics or Quotes?

I've wondered about the  internal monologue considered the key to a great story.  A pretty clear answer I found in Stephen King's famous On Writing. He describes here the writer’s toolbox. Vocabulary, punctuation, grammar, showing not telling  are tools to be found in the top level of an author's toolbox.

 
Internal monologue - the thoughts, dreams, stream-of-consciousness stuff inside a character’s mind only the reader is privy to -  is a tool in the bottom half of any author's toolbox.
It happens through all points of view – first person, third person, omniscient.
 Many people  think in at least partial sentences, talk themselves into and out of all kinds of things, they weigh consequences, wrestle with past experiences all in their head in a split second.
Without internal monologue, understanding what the character is struggling with – how they arrive at their decisions – what motivates them, it’s very difficult for a reader to connect emotionally.
But what should we use in rendering internal dialogue - italics or quotes?

Internal monologue is used by authors to indicate what a character is thinking.
Direct internal monologue refers to a character thinking the exact thoughts as written, often in the first person. (The first person singular is I, the first person plural is we.)
Example: “I cheated her,” David thought, “but maybe she will forgive me.”
Notice that quotation marks and other punctuation are used as if the character had spoken aloud.
You may also use italics without quotation marks for direct internal monologue.
Example: I cheated her, David thought, but maybe she will forgive me.
Indirect internal monologue/dialogue refers to a character expressing a thought in the third person (the third person singular is he or she, the plural is they) and is not set off with either italics or quotation marks.
Example: Lara wondered why David would think that she would forgive him so easily.
The sense of the sentence tells us that she did not think these exact words.

Refining your story's internal monologue is best taken care of in the editing process, but there are a few things you can do in the first draft to help guide what you put in and what you don't:

 -  Leave off the "he thought" or "she thought." If you're deep in your POV character's head, there's no need for it.

 - Use italics sparingly. Many new writers are under the impression that internal monologue should be italicized, but that gets old quickly.