no. you'll be fine. the music files & library (the database that stores info on what tracks you have and where they are) are stored in a different place to the program/application files and are (largely) untouched by upgrades
At worst - and i think this is an issue some people have had with iTunes 6.0.2 - it re-names the existing library file as "previous" and makes a new version of it (presumably with some extra 6.0.2 info in it).
you could always make a backup of the music > iTunes folder to a data dvd if you're worried. (not a bad idea in any case)