OSX 10.4 = Font's: The ones from the System at the top that do NOT have a language next to them (Geneva, Monaco, etc...)
Basically any font in your main System/Font folder other than the foreign language fonts are required by either the System or some Apple-installed apps.
As far as I know, the one you should NEVER inactivate is Lucida Grande if you want your system to keep going! Otherwise, Helvetica and Helvetica New are also needed by some apps.