Well, it depends partly on when you are. When you are in the history of a word, that is.
Take a fairly recent example, I started off e-mailing people, but now email them. The same sort of thing has happened to the words pigeon-hole and hyper-link.
Sometimes the hyphen disappears in another way: for example, fig-leaf is now usually fig leaf, and ice-cream is now often ice cream.
Is there any rhyme or reason in these changes? Only that English words seem to abhor a hyphen (about 16,000 words lost their hyphens in the 2007 of the Shorter Oxford Dictionary). Except, except, except...
...are you talking about two-hundred-year-old ladies, or two hundred-year-old ladies? It that a man-eating snake:
or a man eating snake?
Or, as I saw recently, is the new, possibly dementia-preventing drug solamezumab an antibody drug or an anti-body drug?
I'd be happy to take one, but you're not getting me within spitting distance of the other, I can tell you.
Word To Use Today: antibody. This word comes from the Old English bodig, and is related to the Old Norse buthkr, which means box. The anti bit is Greek.