I seriously doubt these 100's of engineers are checking all the props individually. That's my job and my sample has not been perfect.By mechanic I mean I look at everything mechanically.
I can build a house, work with sheet metal, electric, plumbing, quads and most other things.
I'd consider balancing props on a homemade quad with aftermarket props - but the QC on the Phantom props has proven itself (IMHO) to be quite good.
I do have experience with other quads and how the props vibrate and look when they are out of balance or the shafts are bent, etc - that's a different matter.
DJI has 100's of engineers - many of them aeronautical specialists. If they wanted us to balance the props they'd probably say so.
The engineers also designed the shell that has worked pretty well most of the time. I added Strong Arm reinforcement plates to help distribute the loads. Maybe not necessary, but it can't hurt and might help.