At least in my case...I never use the same css on multiple sites. I might use another slightly altered version, but never pulled from exactly the same file. Precisely for the reason you mention...it would be far too easy to forget a particular nuance from a previous site and blow something out. Very good tip pointing out not to use them across multiple sites Goonie
Since css are small in size, it's no biggie to store em in a folder on your server. And for a large site, they're almost indespensable...allowing site wide visual changes with one file change.