Changing Tires

I've done it both ways and saw no difference in the outcome. You either have two stable frame to ground connections on the front or you have the front sitting on the trucks suspension. Either way you are barely moving the trailer frame as the suspension will allow you to jack one wheel off the ground without lifting the other. You are not going to bend the frame doing that.

I am pretty sure you will get answers why they / we do it either way..

I always hook up to the truck to do that type of work... far less chance of a catastrophic event happening.... And only take a few seconds to hook up