Covering Air Nozzle vs not covering it
 

I've always heard that dry-firing an AEG will increase the chances of it breaking, or cracking, and covering the air nozzle will provide an air cushion and reduce stress on the gearbox.

Has anyone confirmed, or has any input about this?

Yes, covering the air nozzle (lightly) with your finger will simulate a BB in the barrel and cause the piston to not slam into the cylinder head as hard.

Covering the air nozzle? Theoretically yes but it's marginal to reducing the pressure in the gearbox. The piston hitting the cylinderhead exerts immense force and vibration on the gearbox. Covering the nozzle may block the air flow to provide resistance for the airflow and to provide resistance to the piston only momentarily reduces the amount of energy transferred to the cylinderhead.

This is why sorbopads can sometimes reduce the amount of stress on the gearbox and why if bb's jam you might also cause the gearbox to seize because the nozzle/tappet plate is stuck or the piston gets stuck.

It was the difference between breaking and not breaking on ye olde marui shells when upgraded to 380fps.
Not quite such a mandatory requirement anymore.

It makes no difference. In fact, blocking your nozzle enough will cause you to strip your piston.

On semi auto pulling ONLY once every few seconds, blocking the nozzle completely can provide a valid test for air leaks. If your piston still smacks forward with the nozzle blocked, you've got air leak(s) somewhere.

On full auto, you strongly run the risk of the sector gear coming around to pick up the piston before it has fully returned forward which can cause some bad crunchy things to happen because the teeth don't mesh right.