You can only do it for images that the activation server is willing to keep singing for you. Sometimes they go back one or two versions (or a version is know to be jailbreakable, they won't allow it to sign).

The funny part is that the signature is always constant for your device for each version so if you proxy your signing request and cache it (like use Cydia's "on file" thing) you can install and active for older builds.

You may have reverted to older firmware in the past but that's because either you have an older phone (3G and below have no signature checking), did so before Apple started signing firmwares (I believe it started somewhere in 3.x), or you're caching the signatures using TinyUmbrella, Saurik's caching server, SHSHit, or something similar. Apple DOES sign firmwares now and generally refuses to sign older versions soon (couple of hours, maybe a day or two) after a fresh release.