OK, how can I not respond when I add 'Cheers' to many of my signoffs?
The following is from A Dictionary of Slang by Eric Partridge, published by Routledge (new 9th edition just out -- shameless plug, I work for Routledge).
cheers! Often Three cheers! A colloquial expression of deep satisfaction or friendly approval: from ca. 1905, replacing cheerio, a drinking toast; by 1950, and still, mid-1980s, perhaps the most common salutation in raising the glass. It has become, since ca. 1960, much commoner than cheerio as a farewell also, and is often used, moreover, since ca. 1970 as 'Thanks!' as acknowledgement or for a small favour. All uses colloquial.
I use it all the time in speech and writing mainly bacuse that's what my old man did all the time when I was growing up.
Cheers
M.