I’m an editor and my friend is a writer. She believes that the rules of this: http://www.sfwa.org/archive/writing/format_betancourt.htm are necessary for submitting manuscripts.
Apart from when I volunteered at a small press, everything I’ve seen as an editor was first cleared by an agent, but almost none of the manuscripts followed these exact rules. Do all these rules really matter in an electronic world? Is it more important when sending to an agent?
Of course, I’m NOT advocating the use of extravagant fonts that aren’t default, but I haven’t seen anything in Courier. Of course, publishers and agents need an idea of the word count, but they don’t need some special formula of ‘the amount of space a story will take up when typeset’ because the MS will go through many changes and the total word count will be totally different. And using spaces or tabs instead of alignment will most likely irk the layout people, but does it have to be exactly double spaced when first submitting?
What’s all this mean in the world of the agent?
Your friend has, perhaps, not seen the little copyright notice on that web page - it reads '1997'. A lot has changed since then, mainly the fact that a lot of submissions happen electronically - if not by author to agent, then by agent to publisher.
Personally, the only font I can't stand is Courier, and if a manuscript comes in with Courier then I usually change it - because I don't expect the author to intuit that I hate Courier, and also because Microsoft Word lets me. That's the wonder of the new-fangled age: we have options. When submitting manuscripts to publishers I will usually send them in the font they came in - unless it's Courier - and I'll only reformat the line spacing if it's single (1.5 or 2 line space is nicer on the eye when reading electronically). Not a single publisher has ever protested that they didn't like the formatting or font because they probably do what I do and change it if they don't like it.
The only formatting guidelines that matter are the ones set by the agent or publisher to whom the author submits a manuscript. If there are no guidelines that you can find, presume that there are no guidelines and do what you want. The agent cannot then say 'but I wanted it in Courier'. If there are guidelines, they should be followed. It's annoying that we all don't have uniform guidelines, but there's nothing to be done about it - everyone has different tastes (such as 'I hate Courier'). And if you want to know why I really believe that authors get hung up on formatting, read this post.