creating library question

I have been creating libraries by importing a midi file.  I notice sometime instruments imported are correctly voiced. (a guitar part plays a guitar instrument). On other midi files I import, instruments are defaulted to a piano. For  instance drum parts correctly play a drum instrument, while guitar, bass, etc are played by a piano. Is this a function of the way some of these  midi files were originally created? (These are MIDI files I have downloaded off Net, from a wide variety of sources. 

I have been using 'Pool - assign instrument" to get the proper sounds.  When I create a library from some of these files, should I go thru and audition each section (lower box of library) - once I save these should come up correctly?

Thanx