Usually when I compile my scripts from source I never use any compiler flags. After taking a look at this article I found

http://www.zrinity.com/developers/apache/apache2src.cfm

I noticed that they were using the CFLAGS=-O2 option. Is this compatible for fedora core and/or centOS? Do all gcc compilers have this feature, and if they do, is it safe?

Should I use it or should I avoid it?

It's pretty common - if you optimize at -O3, you risk it having the opposite effect, but -O2 is safe and effective when used as directed. You could check the... I guess it would be source rpms... and probably find that fedora uses that already.

i always use benchmarking and find the best optimization flags for my system.
the results can be remarkable.
example:
gcc -O2
tower of hanoi runs in 20 seconds
ggc -march=athlon-xp -O3 -ffast-math
tower of hanoi runs in 14 seconds

thats a 30% increase

overall those flags give about a 13% speed increase

it is very rare for optimization flags to cause problem but it does happen occasionally and is obvious and easy to correct. you just have to compile the app again without them.

Ok, but for compiling something from source, not rpm source, does fedora core already compile using that option by default?

RH/Fedora is the one major distro I've always avoided, so I don't know anything about it. If I understand your question correctly, you want to know how fedora builds from source in order for it become an rpm in the first place? I don't know but searching the web should turn stuff up. For instance, this may help, though it looks old. It says "the default RPM build environment is located in /usr/src/redhat". And the man page for rpmbuild on the web says
so that may give you some idea how rpms on your system would be built by default. I suspect that that would reflect what fedora does itself, though not necessarily so. And I'd be quite surprised if they didn't use some form of optimization.

Well my question was about building from source only. Therefore, nothing installed by package (like rpm).

So what I was asking is that does centOS (which is basically the same as FC) compile using CFLAGS=O2 by default, or do you have to set it up each time?

most source build systems do already incorporate -O2 as default
some packages even use -O3 as default.

i doubt if fedora changes the CFLAGS/CXXFLAGS away from what they are set to by the package developers

one of the most interesting gcc flags is -march
it allows gcc to create code for your processors specific modern instruction set.
otherwise you are by default using an instruction set for processors made 25 years ago.

-march for obvious reasons can't be included by default in generic distributions

also when you set CFLAGS or for c++ code CXXFLAGS as a shell variable the values are added usually in addition to other flags needed by the package.