Plenty of people have loosened their axle nut while the wheel has been on the ground with no problems. However, the Subaru FSM recommends having the wheel off the ground to avoid doing damage to the wheel bearing. It's easy to remove and tighten the axle nut with the wheel off the ground. Put a screwdriver in the rotor slots through the brake caliper and crank away.
FWIW the Subaru techs at the dealership I frequent when buying parts often just impact the axle nuts as tight as they can get them. I just get them as tight as I possibly can with a breaker bar. My torque wrench only goes to 100 ft lbs, and then I bust out the breaker bar and try to go to what feel like 150 ft lbs.