It looks like the short answer is as Aamir said:
Apple stops verifying old updates usually soon after new iOS version is released.
So going backward isn't really possible. Except, if you had the foresight to use a program like Tiny Umbrella which allows you to get the SHSH signatures (and later spoof them) for the devices you own. In my experience it could only download the device signatures for your device for the current most recent OS (and perhaps one version back if it is close enough to release), so you'll have to make sure to get the SHSH signatures for all your devices each time a new version of iOS is posted -- You can then go back to any of the versions you have archived for the devices you have by downloading the appropriate IPSW from a site like this one and use Tiny Umbrella to spoof the apple TSS server while installing using the option key through iTunes.