I'm going to guess here but a large majority of trailer axles are being furnished by Lippert or Dexter. Both would have about the same assembly techniques and there isn't much wasted with grease or accurate bearing torque values.
If I bought a new trailer off a dealer lot or even at the factory, the hubs would be pulled, bearings and brakes serviced, and the whole reassembled with attention to detail. It would be worth it to me to know that the bearings were lubricated and set correctly, the brake parts all aligned and any lubing done to facilitate good brake operation. It's something that only takes a couple hours and can then be peace of mind for many miles.