When using something like Creative crystalizer, I don't think it matters, because that alters the sound anyway.
To answer the question though:
Unless you have some 24bit audio files, then it's almost certain the music will be 16bit and 44.1 or 48k. Ideally it should be set to that.
Some people just put it highest as possible, because they think it will sound better. 24bit is higher than 16bit, so must be better right?
Well no. If the music is 16bit and 44.1 or 48k, then setting higher will not improve the sound.
Ultimately though, for the majority of people, I don't think it would matter what is set to.
