HA - couldn't resist! The reason for live axle trucks/cars having staggered (one front, one rear) shocks is to help distribute and counteract the torque twist being applied to the axle by the driveshaft. A trailer won't have that problem. While remote, I can imagine that the shock of the wheel, hitting a bump, would tend to push the axle up and slightly back - even though mounted front and rear with the spring, there is some give fore/aft. Perhaps the shock might bind a bit if mounted to the back? If the shock was trailing, I think there would be less torque applied to it, and travel up/down might be smoother.
However, if it was mounted to the rear orginally, how could you go wrong?
Of course, this is just a guess,