Back in my graphic design days, one of the most frustrating problems that I had to deal with on a regular basis was wrangling fonts. It's gotten much better than it used to be, but there's still times when you may end up with Safari displaying some text as question marks, or apps crashing when you try and use a different font. Once you've gotten yourself into this situation, it can be notoriously difficult to get back out of it.
Kurt Lang has a very thorough and regularly updated treatise on working with Mac fonts over at his website: http://www.jklstudios.com/misc/osxfonts.html
It should be required reading for anyone who ever has the need (or even just the want) to install or remove fonts from your Mac. Get yourself a cup of coffee and clear some time on your schedule—it's a complicated topic, and Kurt goes into excellent detail.
If my years of dealing with fonts have left me with anything (other than stress-induced skin rash and alopecia), it's this: avoid HelveticaNeue like the plague.