The holding torque is basically the same as the moving torque. I was going to write that once you get a valve set to the position you want, it will stay there on it's own and you can tell the stepper controller to disable the motor. BUT, you will likely need to make a lot of fine adjustments during use, so that option is not worth the bother.
Consider using timing belts and corresponding pullies to connect the valves and the stepper motors. Much more compact and much more forgiving in alignment.
Paul