Bhing raised two good points I'd like to comment on.
First, a recommendation to stop exactly on the centreline - fully agreed. Almost always, the runways slope laterally away from the centreline at a gradient of at least 1%, which is enough to clear rain pretty quickly, and it'll do the same with fuel. If you do have a tank punctured, then fuel will definitely flow away from the fuselage. You can genuinely stand on the runway, and watch liquid flow off.
Secondly, stopping in the middle of the runway does make it easier for the fire crews. They can then drive up the runway and deploy around the aircraft.
Fire trucks are very heavy and can struggle on soft ground. They've got the power and the all-wheel drive to grind across the grass if they have to. (We've got 6 fire trucks, and I've designed plenty of roads for them). The crews often have the training to do likewise. But the grass beside the runway might be softor muddy. The true life stories are legion - about what can go wrong can go wrong can go wrong in the process. Best to keep it easy and simple.