Originally Posted by TerryP
In my opinion, that rules out most people who lack commit access to the FreeBSD src ;-)
Not really; the caveats are actually pretty well documented. If you can read enough documentation to successfully configure, say, an Apache server, then you can read enough documentation to setup your CFLAGS without breaking your system. Once you start reading, and having a good working knowledge of GCC's optimizations, you realize that there's not much benefit to modifying the defaults though, unless you've got specific tricks up your sleeve. Most of the time, i386 and amd64 users aren't going to get any benefit out of using non-default CFLAGS without risking breaking things. On other architectures, there are some optimizations that may help.
All that said, don't be afraid to actually read the documentation and become smarter for it regardless. If you want to use your compiler to its fullest, learn how. GCC has a pretty good man page. Caveats of different optimizations on the world+kernel tree are documented, many of the big caveats are documented right in /usr/share/examples/etc/make.conf
Don't rely on fear to help you configure your system, instead, educate yourself and determine your benefits in any given situation. That's the UNIX way. The notion of "don't touch it, you might break something, and you're too stupid to understand why" is a very Microsoftian attitude.