I always tried to make it as plain text as possible. That is, single-spaced, extra line between paragraphs, no indents, no smart quotes, etc. You want to make it as easy as possible to read even if they're on a smartphone or whatever and this is the safest way how.
That said, I ditto zegota in that agents probably won't mind too much as long as it looks presentable. If agents do have very strict preferences they'll mention it in their guidelines.