If you’ve upgraded to Mac OS X Yosemite (10.10), there is one important piece of advice I can give you as it pertains to fonts: Don’t ever, ever, EVER move or delete HelveticaNeueDeskInterface.ttc. You will completely hose your system, requiring a re-install of the system, or some work with a recent backup. Either way, it’s just not worth messing around with.
Unlike Mavericks (10.9), Yosemite does not have a fall-back option when it comes to the main system font. Desktop icons will use Helvetica, but menus, dialog boxes and the rest of the interface will simply be blank and unusable.
The following are the require fonts for Yosemite, and should not be moved or deleted from the System/Library/Fonts folder:
All other fonts can be safely moved or deleted if you wish, though some may be required by other app such as Pages, Keynote, etc. But in general, those fonts are found in the main Library folder, not the System folder.