There is no exact answer to this question. If you have a very good frame reinforcement, you could be OK to run a steering box brace, but you also probably don't need it. I don't know that we'll ever have enough info to prove that one way or the other. Everyone needs to make that determination themselves. A couple of things are clear.....you need frame strengthening around the steering box, inside and outside, and it's not a real good idea to run a steering box brace without good frame reinforcement.
My personal experience is that I've broken the steering box off the frame while running a brace, and the frame was reinforced both outside and inside at the time. I did mine myself because I did it before either the SBS or C-ROK where available, and because my frame cracked everywhere.....bottom, outside, inside, and across the top.......and I ran a brace. When the box broke off the frame two of the bolts sheared and one of the ears broke off the box, and the frame wasn't damaged. Mine broke because I hit a big rock hard while not paying attention, and it ruined a recently rebuilt box by Tommy Lee.
I also broke the track bar mount off one time, and I was also running a cross frame track bar mount brace. The point is that whether you run cross braces or not, nothing is as important as good frame strengthening.
What Mark is taking about is the steering brace and track bar mount brace, not a good front crossmember reinforcement like Rigidco provides. I like their front crossmember, since it strenghtens the frame. I have beefed my crossmember up myself........but I'm probably the king of cracks.