Hello,

My question is short and I guess simple for more advanced users.

Should I delete older version of library after upgrading it to the newer version? I have upgraded glibc and gtk+ and deleted older versions, after rebooting my box some of the applications didn't want to start ... so I am little confused now ... Can you help me?

Probably not, unless your sure nothing is compiled against the old libraries.

for the most part, if I upgrade something like glib, gtk, atk, etc.. I like to place it back in /usr.

For glibc, ya gotta read the docs. the normal procedure for installing glibc is to overwrite the old one, not
install the new one and delete the old one ( the both use the same files, so when you removepkg, it's gonna pull some of your new files out you just installed)

between your two post here, it might be better to upgrade to 10.0 or 10.1 otherwise your gonna be compiling for awhile to bring slack 9.1 up to date.
just my $.02

How do I know which library should be prefixed to /usr or some other directory?

Well, I'll tell you how I do it, but this is mostly personal preference. when I compile a new lib/application, I look in /var/log/packages to see if it is already installed on my system. If it is there, then I replace it, If its not, then I (generally but not always) install to /usr/local. It really doesn't matter in the long run all that much, just try to keep things together ( like gtk, glib, atk, pango. etc... all in the same prefix, kdelibs and apps in the same prefix, etc...). As for glibc and gcc, I don't replace them unless I am forced to. They are not that hard to do, but sometimes you can run into more problems than what the upgrade is worth.