If you have just done multi-track recording, I was told that mixing the file to MP3 is not a good idea, because it doesn't give the audio the best quality it could have. Is this true? If so, which file type would you suggest mixing the tracks to?

.Wav or AIFF, although for simplicity, Nothing is wrong with MP3. Unless you've got **** hot monitors, you really have to listen hard to hear the difference.
Quote by Jody LeCompte
"My personal preference has always been for a rack unit. One less thing to play dance dance revolution with on your pedalboard"

Are You a PROG-HEAD? I am.
MP3 does suck because it compresses the living daylights out of music. Of course if you haven't mixed the music that might be a good thing. But it's really down to preference. I use WAV (as does everyone else) for recording, but for giving tracks out to other people MP3's handier.