Since a brake bias valve limits the amount of pressure applied to the brake line it is inserted in (ie, decreases hydraulic pressure), you are decreasing braking effectiveness for that brake.

Dave Dewhurst's comment is right on target: since on most cars the rears lock up before the fronts, you want to decrease the braking power in the rear to the point where they won't lock under maximum braking.

I have seen some people partially address the problem of premature rear lock up by putting less effective pads / shoes in the rear (ex: Hawk Blues in the front and stock pads in the rear). This is a bit imprecise, but an easy fix if you are in a bind (no pun intended).

Have you run the car yet? Do you know if you have a problem with the rears locking up before the fronts?