How to safely clean out /lib, /usr/lib?

I have a metric buttload of ancient libraries in /lib and /usr/lib, some 
probably going back to the original FreeBSD 3.3 installation on the 
machine.  I've run libchk to get a list of unreferenced libraries, but I 
wanted to ask before I throw myself to the mercy of my backup tape: is 
there any reason not to delete all of those old libraries?

On the same subject, I've periodically gone through {usr,}/{s,}bin after an 
installworld and deleted all commands more than a month old or so, the 
logic being that any commands not updated by world are remnants that can be 
purged.  Is that a reasonable assumption?
-- 
Kirk Strauser