i almost always tie low to ground.
My two cents on this. I am no expert, but I´ve dabbled in electronics for 30 odd years and has done my share of mistakes.
Now, depending on circumstances that can be a bad idea, or a requirement. The situation is as follows.
1 - the output has two amplifiers.
One sends the "hot" pin 2 signal, another the "cold" pin 3. If you connect the output of pin 3 to ground, you are putting the output of the "cold" amplifier directly to ground. It will most probably not be destroyed (see, I said most probably), but it will draw a lot of unnecessary current. A bit of heat and a bit of extra drain on any battery may be the effect.
2 - the output has a transformer.
Now, in budget and modern equipment we seldom see transformers on the outputs. They are quite frequent in field mixers used for film or TV work though and in vintage equipment they are quite common.
The transformer output is electrically isolated from everything else. Therefore you have to connect both ends of the wiring, meaning that to get any useful signal you have to connect "cold" pin 3 to ground.
There is a multitude of configurations out there, where the manufacturer knows that people are going to connect pin 3 to ground (regardless what the manual may say). These generally are sort of "pseudo"-balanced inside the box. The mini-MP is one of them. My guess is that it is better to leave "cold" pin 3 unconnected for those as well, but it surely depends on the exact circuits.
Gunnar