On your computer I mean....any specific file types?
I rip most of my stuff into a format called WMA Lossless (you can do this with Media Player 9 or 10) and it's been proven that you cannot tell the difference between the cd it came from and the file, even though the file is about 10MB less than an uncompressed WAV file. The bit rate is variable and changes as the music does so some parts have higher bitrates than others.
I don't do much in terms of mp3 files since Media Player has actually corrupted some of them where the duration is terribly incorrect and I can't browse through the song anymore. I've yet to see it do such a thing to one of it's own children. Also, believe it or not, mp3 encoding sounds different from the cd...not everyone can hear the difference, but it's there so I don't do that anymore.
What also helps is I don't have an mp3 player, nor do I want one and since Media Player can burn cds why should I give up quality in the first place when a cd can only hold 80 minutes of music any way you slice it?

So what do you guys do?