iTunes doesn't need to convert mp3s. Once a file is mp3 - it can play natively in iTunes. What you may be thinking of is AAC format that is proprietary. When ripping music from a CD, Apple's default is to encode in AAC, but you change this in you preferences area to mp3 (and choose your bitrate). You may also be confusing mp3 with wma (windows media audio). iTunes cannot play native wma, but can convert to AAC for you. Thusly, keep everything in 256k encoded MP3 format you won't go wrong - plays everywhere and sounds great.