Hello daveroo,
The null zones introduce unstability in the aircraft control. The best is to have a zero null zones. But with the older aircrafts with mechanical control link to control surfaces, as the aircraft became older, the null zone may increase. With the newer aircrafts, the black boxes, eliminate pratically the inflence of the null zones even if physically theree is some dead zones in the controll links. In any case, a well maintained aircraft will have almost not perceptible null zones. So take care of your virtual aircraft and put to zero the null zones (dead zones) That is my suggestion.
Have a good day
Benoit