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.
(Unless you use Cydia's "on file" service, which caches the latest firmware's "okay, install this" response whoever you open Cydia for restoring later.)
Everyone with a jailbroken device (~10% of users) is signed up, as well as others who use it just for the ability to downgrade -- useful for testing old firmware revisions for development purposes, for example.
(Seriously, the scale is quite impressive. There are many millions of devices with these "SHSH hashes" saved.)
I've seen you cite that number, 10% of users being jailbroken, a few times. Where does it come from? Have 18.7 million people really jailbroken[1]? That seems high to me. Is it possible that the number is (somewhat) inflated due to people jailbreaking multiple times (i.e.: upgrading and then jailbreaking again)?
a) That number comes from Flurry, the leading iPhone analytics company. For further context on the history of that number (with more accurate renditions and exactly what was last stated to me from Flurry), please read this:
b) I am not even certain how I would count the proposed flawed number: I track unique devices by UDID that see my server, and yes: 18.7 million people is an easy number to claim are jailbroken. 1.5 million of these devices use Cydia every day, and 10 million of them every month (edit| damn it: I said "week" here at first; 5 million per week).
Seriously: jailbreaking is popular, and 10% seems like a "non-issue" number when you go around and start asking random people "have you jailbroken your device or know people who do?".
So far, my favorite story of this is of a cab driver I had while going to a conference in Vegas. She had an iPhone, and was clearly very very non-technical. When asked if she had jailbroken her phone, she didn't even know what that /meant/, and when I explained to her that I had developed "Cydia", she hadn't heard of it.
However, later in the conversation, she started mentioning how much she hated AT&T, and was glad she was using T-Mobile, at which point I realized that all this time I had not been asking users enough questions. I asked her if she was, in fact, using T-Mobile on her iPhone, and then answer was "yes": she had bought her phone from an unlock parlor.
After a few more questions like "do you have a brown icon on your phone that looks somewhat like the App Store?", I realized that she in fact had a jailbroken/unlocked iPhone with Cydia installed on it (and she had even used it a couple times), but was simply not technical enough to recognize that state using any of the terms that I normally have access to while conversing.
I am curious how you are checking, and what your service is: you must have some kind of correlation. (Also, please read my above response, which you will not get a notification of from HN.)
