On iOS 5, they figured out a re-restore method even for A5 devices but way late in the game (I think not long after iPhone 5 came out). Re-restoring is another exploit process of its own since Apple is strict about firmware signing.
The reason iPhone 4 can be re-restored on any version is because of limera1n. As much as you might not like George Hotz personality or ego, he is responsible for this. This is a hardware exploit, so software version is irrelevant. Regardless, iPhone 4 will always have at least a tethered version of every iOS version even if another untether never comes out.
I wouldn't hold your breath for an iOS 6 re-restore method any time soon.
And once you go iOS 5 -> iOS 6 on A5 devices, the re-restore thing is no longer any option (i.e. moved to 6.1.2 from 5.1.1, even with blobs, you can't jump back to 5.1.1... only exception being 4.3.3 re-restore on iPad2 but that is moot imo)
If you are on 6.0, you can only go to the latest version as of signing which now is 6.1.3. If you're on 6.0, but still have blobs for 6.1.2, *IF* a re-restore method comes out you may be able to jump to it.
Originally, the iOS 5 re-restore exploit was only supposed to be able to re-restore for the version you were on (if you were on 5.0.1, re-restore only to 5.0.1) but through their research they figured out a way to re-restore any iOS 5 version to any iOS 5 version, provided you have blobs.
So if you were on 6.0, I'd recommend staying there and enjoying the jailbreak until the next big release comes out. It's not like there's any feature updates anyways, it's all bug fixes.
I hope this makes sense, and somebody correct me if I'm wrong. I follow jailbreak details way to closely
JB'er since first gen, ten minutes later out of taking it out of the box.
No jailbreak, not interested. I bought iPhone 5 and waited it out, but even with nice hardware, NEVER again for me.