I'm sure many will chime in here, but my "logic" says that the whole weight
normally is taken by the axle
(and the tongue jack when used). So the bending forces would normally force the frame into an arc bowing up in the middle. If the frame was totally suspended by four jacks at the extreme corners the frame would describe an arc bowing down in the middle, sagging sadly and leaving the wheels hanging loose and forlorn.
No, wait, it isn't as drastic as all that and these arcs may not be visible at all. In fact on my Burro's super sturdy frame there is zero bending (sure, tell me more lies). Well, negligible and unnoticeable to my old eyes anyway.
Ideally I would think that if you divided the length of the trailer into thirds the best place would be one third from each end in order to distribute the weight
as evenly as possible. Probably much better distributed than on that one lonely axle
. There may be other considerations, but I putting the jacks at the extreme corners is probably the most stressful , especially if you jack it up so that the wheels are not even touching the ground.