I do, of course, basically agree with both SirReal and Kim; however (isn't there always a 'however' from Dusty?!) ---
there are times when used as a noun that adding the hyphen makes sentence structure clearer. A 'sign-on' can be some kind of codde, passord or whatever that people may use to sign on to some system with, and 'sign-off' is often used as an informal way of saying 'closing formula' either for things like letters, or in more figurative contexts.

I think in these noun cases, the hyphens are at least sometime justified.

"at sign-on" = "at the time of signing on"

Analagous with 'start-up' and 'switch-off', etc. when used as nouns

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Note added at 15 mins (2004-09-22 12:08:16 GMT)
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Oops! That should be code, password and sometimes, of course!

I'm only aware of these terms as verbs. I'm going to sign off now. A hyphen is used for compound words: mother-in-law, adjectives before nouns as in "a home-grown tomato",

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Note added at 17 mins (2004-09-22 12:09:37 GMT)
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Another useful \"rule\" for hyphens is \"use hyphens to avoid amibuity.\"
1. He recovered from the shock of losing $10,000.
2. Mrs. Jones re-covered her sofa.

1. Aerobic dancing is good recreation.
2. The novel was a re-creation of his own childhood.

So \"do this before sign-off\" could be the right solution \"to avoid ambiguity.\"