The reason touching the negative cable to the positive terminal on the battery should not be recomended is the same reason vehicle manufacturers give instructions on jump starting a battery with a neutral grounding point, not on the negative post of the battery.
The negative ground system on an vehicle uses several grounding points through out.
The positive leads to various actuators and components are fused and relayed.
Capacitors are simular in fuction and theory to batteries except the capacitor discharges more rapidly and some what violently.
For this reason the method of discharging them is important.
Forcing the electrical charge by looping the negative cable into the positive electrical storage and current flow is alright on some older vehicles.
Newer vehicles are more sensitive because their circuits, components, and actuators use smaller gauge wiring and fuses to protect them, they are more suseptable to failure. Some failures won't be have immediate results, but the stress on the sytems they cause can be evident in the reduced longevity of the affected parts.
Some manufactures suggest disconnecting the positive battery cable and touching the negative battery terminal for the reason as stated above.
I suggested using the negative battery cable as a ground to a chassis which is effective in the sense that it is a larger conduit for the electricity to follow, compared to the smaller grounding points throughout a vehicle when a reversed current flow is used.