Actually
It's a bit complicated.
If all you are hoping to achive is more complete transformation from austenite, then I would first check your quenching and tempering practices against the steel you're using.
If the steel should have a MF point at or around room temperature, then cryo isn't going to help you make more martensite.
You may have a problem with hardening completely if your soak time at austenizing isn't long enough, or if your quench is inappropriate to the steel.
Assuming all of the above is in order, then cryo is really only called for in some of the more complicated alloys. Basically, elements like Chromium, which can push off the pearlite nose later in time, helping make the steel more deeply hardening can have the unintentional (or, i guess, sometimes inpetntional) effect of suppressing the MF point. Sometimes to the point where it is very low.
In this case, you will find that the soner after quenching you do the cryo the better. If you wait too long, a smaller percent of the retained austenite will convert, as it will have time to stabelize.
For maximum success, in theory, it should be done before a temper, because a return of heat up to 4 or 5 hundred degrees can stabelize the retained austenite.
In practice, however, this is dangerous, as it can result in cracking and breaking, and other not happy events with the steel.
Thus it is usually reccomended that a 200 or 300 degree (f) "snap temper" be performed prior to cryo. This helps to relax some of the high stresses in the newly formed martensite, but does not stabelize the retained austenite to any appreciable degree.
Also, check into the TTT curve of the steel in question pretty carefully if you can. Even if the MF point is below room temperature, or even below zero, that doesn't neccessarily mean that it needs a full deep cryo treatment.
On the other hand, however, there are the "side" benefits of cryo processing.
To be fair, I have seen no scientifically CONCLUSIVE evidence that any such "benefits" are at all usefult to the bladesmithing community. A Dr. Freidrich Diekman was on another forum for awhile trying to present the case for cryo. Overall, his message was not recieved very well by those in the know. However, while he was around, he was nice enough to forward me an excel spreadsheet of a decent number of scientific studies of the materials science research on cryogenic processing he thought would be relevent.
While I didn't pursue too terribly deeply, I was both intrigued and disappointed. Most of the papers on the list were written about advanced cermets, or exotic alloys, or aluminum / titanium alloys, etc. Very very few had anything whatever to do with tool steels or simple steels.
The ones that did though, were about tine greek letter carbides and other neat stuff that I won't even pretend to understand. From the looks of some of the research that was presented, there very likely is something to this cryo beyond just martensite transformation that I'm just not grasping.
That is NOT however, to say that all of the "smoke and mirrors" is anything but hogwash. Just because there MAY be something more that I, personally don't yet grasp does NOT mean that any such thing actually exists, or, even if it does, that it would be of any actual use to the knifemaking community in general.
Not being personally familiar with any of the steels in question, I would suggest you get ahold of the manufacturer. Crucible has some real crackerjack metallurgists working for them, and one in particular (I think his name was Dave, he works in the Texas office) is extraordinarily happy to talk steel all day long with even hardheaded amatures like me! He's their stainless specialist (so he might be of some serious help, at least on the S30 V issue) and knows his shiznit!
