There have been discussions on this forum about guidelines for various social media options, below are links and content from three different reputable sources regarding Twitter use. Check them out and post some thoughts.
UK Govt guide to Twiiter
http://www.scribd.com/doc/17313280/Template-Twitter-Strategy-for-Government-Departments
Mashable Twitter Guide Book
http://mashable.com/guidebook/twitter/
And from the Wall Street Journal, an excerpt on Twitter use from an overall social media user guide.
“Let our coverage speak for itself, and don’t detail how an article was reported, written or edited.
Don’t discuss articles that haven’t been published, meetings you’ve attended or plan to attend with staff or sources, or interviews that you’ve conducted.
All postings on Dow Jones sites that may be controversial or that deal with sensitive subjects need to be cleared with your editor before posting.
Business and pleasure should not be mixed on services like Twitter. Common sense should prevail, but if you are in doubt about the appropriateness of a Tweet or posting, discuss it with your editor before sending.”