One of things I love so much about working with carbon steels is the number different characteristics that can be imparted by thermal treatments. There's just so much you can do with this stuff if you have a good handle on what's going on inside the steel.
Mine are some of the ones where you'll often see a 90 at the back of the ricasso. For clarity I refer to that area as the guard stop. To date I've not had one break in this area, even when I'm doing destructive testing. I'm only entering this discussion because quite a few pics of my knives have appeared showing the guard stop, and I can see that it might cause some confusion.
I'm guess I'm probably becoming noted for doing things that are somewhat outside the box, such as solderless guards, scale-free heat-treating of carbon steels, drilling pin and bolt holes after assembly, and so on. There are a few other things I do which I haven't documented, that might cause looks of confusion. But I do test quite a bit, so I'm not just running blindly out there.
Let me state that I am not trying to say that my way is the right way. I am not saying that people should be doing it the way I do. In fact, I would suggest that you don't do it this way if you are using air-hardening steels, or have not done enough heat-treating and testing to be confident with what you're doing. This is just the way I am doing it, and it works well for me.
Here's an old shot of where I've filed in the guard stop on a hunter. You'll notice that I've filed a shelf all the way around the ricasso. This is something else that you will occasionally hear should not be done.
Will this type of setup hold up to punishment? Here's a shot of a 10" camp knife being tested. The guard is resting inside, but right at the edge of the pipe. On this knife I did many of the things that I've seen be warned against. The guard stops are at 90, the ricasso shoulders (plunge to the stock removers) are at 90, the guard is a tight fit, etc. There is some serious stress going on here.
Here's a shot of the same knife, and a smaller one (El Camino) which was tested to destruction. These look pretty rough. But there's not much sense in spending hours making one pretty when you know you are about to destroy it.
Now, the smaller knife did break, but that was fully expected. You'll notice that the break was at the start of the clip (which was radiused by the way). You can see the transition line (where the softer steel starts) right behind the clip. There were no surprises here. It broke exactly where I expected it would. The sharpened clips are ground in the hardened steel obviously. Notice that the tang and the guard stop area is completely intact.
I believe that one reason this works so well for me is because the back of the ricasso and tang are at a tough spring temper. I also believe that the distal taper plays an important role. Of course, when you really think about it, even though these are at 90 degrees, there is still a tiny radius. Look at the edge of your files. The files don't have sharp edges. There is a tiny radius.