If possible, unless the spacer is interfering with the bottoming snubber, [upper spring seat] I would recommend installing it on the top for the following reasons:
1. By having it on top it becomes Sprung weight as opposed to Unsprung.
2. When on the axle side of the spring, the spacer experiences extreme pressure moments with every bump, and I contend that shock moment is what is leading to first, fractures, and ultimatly, collapse.
3. When it is top mounted, the spring is reducing the shock moment and turning it into nothing more than a momentary pressure [load] moment.
However, the manufacturer has determined bottom mounting would be satisfactory, so, you must realize you are using yourself as a test monkey.
Don't expect them to be very sympathetic if your modification leads to damage to your vehicle.
I would be more inclined to recommend a different brand lift that utilizes the above rear spring spacer.