If you mount porportioning valve in the cab you can dial all your braking into the fronts and do smokey burn-out donuts just like Jimmie Johnson (or your fav-0-rite NASCAR jockey).
If you watch closely in cockpit shots, you can see them twisting away at the knob after they take the checkered...
So the answer is, you can mount in the cab if you want, and racers do, but if you are only using to adjust once and then forget it, I wouldn't necessarily bother. I'd think a frame mount with access door in the cab floor would be almost the best of both worlds.
