CSS Architecture
engineering.appfolio.com
engineering.appfolio.com
However "Never nest rules purely for code organization. Only nest when the outputted CSS is what you want." is something I find harder to agree with.
If you don't do this then you're missing out on a ton of productivity from these tools and for what? Saving a few bytes in a CSS file that's going to be cached after the first request.
It's a pity we can't use Less or Sass directly in the browser in a reliable manner. CSS4 maybe?
The file size issue isn't really that much of a concern unless you're on a really large site, but specificity issues affect everyone.