I have seen people building trailers and they were lazy and used the steel jig for the ground. It made it easier to move the trailer because they saved a few seconds that it took to move the ground when flipping the trailer over. Once the axles were installed they would flip the trailer back, right side up and finish some welding. The trailer was grounded, from the jig, through the steel hubs to the trailer. The new owners couldn't understand whey the bearings didn't last long.
As far as disconnecting the battery, it is just a precaution that I take. If you don't have any sensitive electronics then it probably isn't an issue. I would rather give advice using caution instead of costing someone a lot of money because they took a short cut. YMMV