Will a locksmith prefer to install a handle on a door that is hung or laying flat on horses?
I would vastly prefer to do them hung.  

As far as just attaching the hardware, that job is a cakewalk when the door is hung.  Doing it while the door is on sawhorses only creates a handling problem; the doorknob will be awkward.  

If holes need to be drilled for the knob, latch and strike, then trying to do that separately using measurements would be insane. 95% chance you'd mount the carefully measured door, and by golly, they'd be misaligned. 

Doing the work hung doesn't add enough difficulty to be worth the trouble it will cause for fitment.