It depends on the document's written style, but be internally consistent.
If speech is enclosed in quotation marks, then thoughts also -- unless there is another means of showing thought content, such as italics; however, italics are out if they are already used to any great extent, e.g. for emphasis.
I have read books containing much dialogue (and unspoken thought) that have used no quotation marks at all. It can be done -- if the writing style is suited -- with no inconvenience or confusion of the reader. However, I'm not sure I'd recommend this style to any but very competent authors / editors. (I don't know how you consider your writing skills.)
If confident it would work, go for the simple, uncluttered look; if not, go with tradition, and include thoughts in " "s.
"Style is knowing who you are, what you want to say, and not giving a damn." — Gore Vidal
Thank you for your help. I was asking on behalf of a friend who has written a pithy political satire type story (of 100,000 words) and is polishing it in the hopes of 'doing something with it'! (I would add that the sentence isn't from the 'book'; I just made one up as an example