When I load a trailer with any cargo one rule I use is when I can back off to the side and the trailer and vehicle sit level traveling seems to be alot easier. One thing can happen is if the tongue is to high when brakes are applied the trailer will try and lift the back end of the truck. If the tongue is to low it will try and push down on the backend which in some cases will make your frontend raise which again you lose braking power.
I've had the experience with a load that was too far back of the axles. Unfortunately I couldn't rearrange the load as it was round bales. I almost lost the whole load/truck/trailer as it got real squirrely on me. A friend was pulling another load of hay behind me and told me that at one point he say the FRONT end of my truck. Lady luck was with me that day.