by Corinne McKay
http://thoughtsontranslation.com/2014/02/10/raising-your-translation-rates-how-and-when/
Short answer: with new clients, and when you’re already really busy.
...
If 100% of potential clients accept your rates without negotiating, you could be charging more.
And here's a neat PROZ page that shows members' average reported rates for all language pairs.
http://search.proz.com/?sp=pfe/rates