sources.list - reasonably stable

[Mark Devin <mdevin@ozemail.com.au>]

I am just a little confused from all the previous posts regarding
sources.list

What I need is the most up to date stuff, including security updates.  I
am willing to risk a little bit of testing but don't want really
unstable stuff that might break my system making it unusable.

Is this an OK sources.list?
deb http://ftp.au.debian.org/pub/debian stable main contrib non-free
deb http://ftp.au.debian.org/pub/debian-non-US stable/non-US  main
contrib non-free
deb http://ftp.au.debian.org/pub/debian testing main contrib non-free
deb http://ftp.au.debian.org/pub/debian-non-US testing/non-US main
contrib non-free
deb http://security.debian.org/ stable/updates main

Are the first 2 source lines necessary - ie will it all be in testing
also but just later versions?
Do I need the last line - ie. will this all be in testing also?

How safe would it be to always run with "unstable" - continually
updating?.  Am I likely to get a broken system which cannot be fixed
without expert skills?

Thanks.

Mark.