Thanks for the replies.
As for the redundancy, the arduino is being used to measure (via a resistor voltage divider) the voltage of the battery pack (usually 11.2v) BEFORE the regulator, as measuring it after the regulator would give me no change as the voltage dropped (due to the arduino reference dropping with it) .
This monitoring didn't work until i linked the negative term on the battery to the arduino's ground - hence the reason for what may seem like a redundant connection.
If it makes more sense, I have 11.2v of battery power, running (via the regulator) a raspberry pi, which is running the arduino.
Sounds like i'll be safe, i imagine any currents heading to ground will take the direct path to the battery pack, rather than going through the arduino -> usb connection -> pi -> voltage regulator -> battery.