From the MLA Handbook for Writers of Research Papers, 6th Edition:
If a quotation runs to more than four lines in your paper, set it off from your text by beginning a new line, indenting one inch (or ten spaces if you are using a typewriter [chuckle–ed.]) from the left margin, and typing it double spaced, without adding quotation marks. (3.7.2)
Verse quotations of more than three lines should being on a new line. Unless the quotation involves unusual spacing, indent each line one inch (or ten spaces on a typewriter) from the left margin and double-space between lines, adding no quotation marks that do not appear in the original. (3.7.3)
Note that this does *not* say: “Double-space everything *except* block quotations, which should be single-spaced.” Apparently this confusion is widespread, judging by the papers I get.
MLA style dictates that EVERYTHING, even the identifying information in the upper-left-hand corner of page 1, be double-spaced. If you’re wondering, “how should I format this block quotation in MLA style,” the answer is: double-space. If , more generally, you’re wondering, “in MLA style, should I double-space [anything]”, the answer is: YES.
According to the MLA, thou shalt double-space.
Now, there’s no especial virtue in double-spacing or not, but if you’ve been asked to use a style guide, then use the style guide!
That is all.
PS: Some other time we’ll deal with the widespread conviction that “quote” is a noun. (I.e., “I’ve got my quotes . . . I’m all set.”) For this post, at least, let’s distinguish actual guidelines from pet peeves!