Ubuntu Installation :: Remove Unneeded Config Files Automatically?

1) Whenever a new release of Ubuntu comes out, I've always done a full replacement (Booted up live CD, formatted / and formatted /home) to ensure there's zero incompatibilities, including configuration files in my /home. That has, finally, gotten annoying and I was wondering what the actual chances of a configuration file causing problems was in the 6 months that Ubuntu was released in. Also, special consideration for Gnome 3 (aka gnome-shell) coming out with the current gnome configs.

2)Any way to remove unneeded config files automatically? I don't always use purge when I remove a package just in case I want to reinstall it, well weeks later I def don't want it, is there an apt or dpkg command that will automatically remove them after the package is gone? I know autoclean and autoremove handle orphans and unneeded .deb's

3) How stable is Sidux in reality? Ran it in a VM and had a few errors every so often while using it, but i wasnt sure how much of that was the fact it was a VM. Some say its stable enough for daily use, some say it breaks every other day