In you would like to rid the cruft on your system, or if you system has a virus (unlikely) or even if you want to install the system again at a later day a good way to do this is to package all your installed emerges. This can be done with a slightly-modified version of holla’s bash script:
Once this is done you can backup your Gentoo system by:
Obviously you might want to do more like
/root. Then you can extract the tar on a new stage3 and
emerge -K --deep world. I’d recommend not trying this if too much time has passed between backup and reinstall as portage configurations may have changed, but otherwise it can save you alot of compiling time.
Please read the comments for other (easier) ways to do this.