Use of Hyphens

[Leasha]

I need some hyphen help please. If the dictionary has two spellings for a word, how do you determine which spelling is the better choice? For example: coworker vs co-worker.

[Celeste Stewart]

Here's what the Yahoo!' Style Guide has to say:
"Generally, use a hyphen between this prefix and a root word unless the word is one that your dictionary closes up (for example, cooperation, coordinate). But always use a hyphen when the resulting word denotes a shared occupation or status. Examples: co-creator, co-host, co-parent, co-star, co-worker."

So, go with co-worker. (Besides I always see the word "cow" in "coworker" and it confuses me slightly.)

By the way, you can download Yahoo!'s "word list" and import it into a spreadsheet. That's where I found this particular entry. Really helpful! Here's the link:
http://styleguide.yahoo.com/word-list