Never open high side Manifold gauge valve?

Both can be correct but only for pulling a vacuum on the system. The way the AC works is simple, you charge the system with a refrigerant that starts as a liquid, that liquid then absorbs heat from the evaporator core and turns to a gas as it does. That gas then goes into the compressor which compresses it and sends that into the condenser core which is the high side. There it turns back into a liquid. Then it goes through the orifice or TXV which meters the liquid as it enters the evaporator side of the system, and the cycle continues. Now while it's safe to open both valves when you are pulling a vacuum, it is not safe to open it when the system is operating. If you notice both videos warn about liquid going into the compressor. If you open the high side valve when the system is charging or in operation the liquid on the high side can get dumped into the compressor, that does very bad things to them. So the SOP when you talk about the system with folks who are not AC certified is to say, don't open the valve. That is because of the above and that valves can leak if not tightened down tight. The AC set I let other folks use has been modified so the high side valve no longer operates. Hope that answers your question.