It's also about the layout.
There are three ways that you can get coupling between the ignition circuit and the monitoring circuit.
One is resistive, where the current in the ignition circuit produces a voltage drop between two points which become part of the monitoring circuit. Essentially, you want the monitoring leads to be directly across the battery terminals - or at least at the battery connector for a model.
One is inductive, where the current flows in a loop and another loop exists in the monitoring circuit and the two loops couple like a transformer. All wires in each circuit should run parallel and close (but separate for the two circuits), at least as "ribbon" or "figure 8" wire, possibly twisted as MarkT points out. Preferably both the (low voltage) wires in the ignition circuit and the monitoring circuit.
The third is capacitive, where the electrostatic field is coupled particularly from the high voltage circuit to the monitoring circuit (note that both the monitoring wires and the power wires to the monitoring system are concerned in all three scenarios ). This is dealt with by keeping high voltage wiring as far away from the monitoring circuit as possible, and interposing a shield (connected to the ground or negative wire of the supply) between the high voltage circuit and the monitoring circuit. One approach of this is indeed, to use shielded wire from the monitoring point to inside the case shielding of the monitoring unit.