This is what I do.
I import stuff of all types into iTunes, and changing the prefs everytime is a pain.
I created Automator plugins for importing into iTunes. One for Mp3,AAC,WAV,AIFF, and Lossless.
whenever I have a file that I want to import, I just right click it>automator, and select the type of method I want for importing. After it imports, I have it set to create a new playlist called, Mp3 imports, or Lossless imports ect..to keep the track I just imported seperated from the library for quick access.
works great.
That's what I do anyway,