Browser testing old versions

What do people do to test earlier versions of browsers? Is there a general consensus of what to do?

Windows 7- Right now I have latest Chrome and FF installed. I'm about to install the latest Safari and Opera, and I have IE Tester (http://my-debugbar.com/wiki/IETester/HomePage) to test all of IE, but what about older versions of the previous 4 browsers?

You can download and install older versions of Firefox from http://www.oldapps.com/firefox.php. They will run but you can’t run different versions side by side. That same site has old versions of Google Chrome but I don’t know how it will behave. Just try it.

Cool, thanks. Is it worth it to test out older versions of safari, opera, and FF? Or for the most part is it only IE and its versions that will cause problems?