You can't really back up the entire OS...this is why saving your SHSH blobs is so important--for most devices, they are what allows you to return to older firmwares. However, I see you are on a 4S, so that isn't going to work for you. Because of hardware changes made for the 4S, you can no longer use those saved "blobs" to restore to an older firmware. If you update to 5.1, you will be stuck there until a new jb is released.
The best thing you can do is be safe: don't run out and install every new tweak or app that gets released in Cydia, and do some research on conflicts before you do install anything. (For instance, the jb app Copic has some serious conflicts with RetinaPad and PhotoAlbums+ that can lead to the need to restore, which for you would mean 5.1--but a simple search would alert you to that before installation.)
That said, you can still sync with iTunes without fear. Just don't update when you get the popup asking if you want to update to the newer iPhone software.(And, obviously, don't do the over-the-air update via your phone either!) You can also save your Cydia apps in a number of ways, but in this situation it's kind of beside the point since you wouldn't be able to reload them if you had to restore to 5.1.