Should the master be left on, aircraft evacuated, and battery disconnected? With fire service present?
My opinion is that if fire is a risk, having or leaving the power on increases that risk. Asking passengers to leave promptly and safely would be a really good idea. You should take reasonable measures to secure the aircraft and minimize risk, unless you feel you are in imminent risk of harm.
Though theoretically possible, a meaningful spark at the master switch itself is unlikely. It only energizes a solinoid (aka contactor) which is where a spark is much more likely if it's going to occur. That solinoid is sealed, so very unlikely a source of ignition. Disconnecting batteries is best left to maintenance staff. Doing that wrong really raises the risks that you are trying to minimize.