In my freelance-editing work, one of the most common mistakes I see in all skill levels is confusion over compound modifiers. Believe me, it took me years before I understood them--luckily I had professional copy editors on my early novels. As with comma usage, it's still an imperfect art, no matter which style book you use.
I lean on Strunk and White's "Elements of Style" when in doubt, but even that sacred tome is fairly vague, calling them "compound adjectives" instead of "compound modifiers." The rule is pretty basic, though. When using two words working together to describe or modify a noun, they should be hyphenated.
A fine example is the title of the Hemingway story "A Clean, Well-Lighted Place." If he had used "well lighted," we might have wondered if the light had radiated from a well, perhaps by someone dropping a lantern down it. You may say it doesn't matter, because the meaning will always be clear, but why not help the reader as much as you can? After all, you have given her a very hard job in the first place, stringing a bunch of glyphs across a page or screen and expecting it to be translated just as you have imagined.
Try this one. You're in a science fiction story (or is it a "science-fiction story"?), and the orbital and rotational periods of the Earth have been altered. But you write "He walked under the late afternoon sun." You might very well mean the sun is arriving several hours behind schedule. But if you use "He walked under the late-afternoon sun," we know the two words are joined. Make sense?