The key is to consolidate one's iTunes Library (.itl) file together with one's iTunes Music folder in a folder labeled iTunes on external HDDs, one for each of the 5 machines. By keeping all the external HDDs synchronized, each time one adds tunes to iTunes on one HDD, all 5 machines will have identical iTunes folders in terms of both the actual tunes and the new iTunes Library (.itl) file.
Later, when updating iTunes, the program will preserve one's playlists, star ratings, etc for the update but also create on the HDD a folder entitled Previous iTunes Libraries in the event one wishes to roll back to the previous library.
http://muvipix.com/phpBB3/viewtopic.php?f=25&t=8074&p=71716&hilit=iTunes#p71716
With regard to the procedure described in the link, note that with the latest version of the program it may not necessary to hold down the Shift key when opening iTunes--if it does not see one's iTunes Library in its usual location, it will automatically ask one to locate one's current library file. Simply click on the correct .itl file and--Voila!--one's library opens instantly.