What's the good way to clean up the system (and is Bleachbit safe) on Ubuntu 14.04?

To remove any packages that aren't needed by the system:

sudo apt-get autoremove

The other answer for this question:

sudo apt-get clean

This just clears the package cache for downloading new packages, it will not uninstall any packages.

See: https://help.ubuntu.com/community/AptGet/Howto

Ubuntu tweak will be to your liking.

It has a "janitor" option where you can clear caches files, delete old kernels, delete orphaned .debs and more like that. But what gets removed tends to be a few MBs of software that is not longer used on your system but will not have interfered with what you normally do.

A crash of kate should be treated as such: search the crash online and see of there is a fix, otherwise you can opt to file a bug report against kate.

To get rid of partial packages - sudo apt-get autoclean

Get rid of unnecessary locale data - localepurge

Get rid of "orphaned" packages - sudo deborphan | xargs sudo apt-get -y remove --purge

You can refer this article for more detailed information

http://www.ubuntugeek.com/cleaning-up-a-ubuntu-gnulinux-system-updated-with-ubuntu-14-10-and-more-tools-added.html.