With the upper and lower control arms, the axle doesn't change angle as it moves up and down. Therefore it doesn't matter if the axle is supported or hanging when you check/fill the differential. As long as the car isn't at a crazy angle you'll be fine.
Vince, I thought he was asking if the fill reading would change based on if the axle was up or down. The axle doesn't tilt forward and back when it goes up and down so your full mark with the suspension extended will still be full when the suspension is fully compressed. But you're right, access is easier with the axle hanging.

