I try not to use CSS that requires special fixes so it’ll work in other browsers (not other versions of the same browser). The same goes with the PNG-fixes for IE6 (and older).
When it comes to validation of CSS; well, some people consider it alpha and omega to not have any errors nor warnings. I still think of warnings as just a warning, not an error. Also, if you want to hide some text with the same colour as the background, I wouldn’t worry about the warning the validator gives. It’s just good that it tells you what’s wrong and what ain’t. It’s just chewing through it’s algorithm.
I’m looking forward to the time when end-users have upgraded their IE to at least v8, and is supporting some CSS3 properties.