That makes sense. A solid axle should be able to be in the air as long as it still holds the vehicle weight.

 

When you're talking about zeroing out an 1/8" of runout across 5+ feet of distance, I think it's more accurate to have the vehicle in it's natural state with all ball joints, etc under their static load. There will be differences in how all the parts are related to each other when unloaded.