The only reason you would have to intentionally vent it if you have overcharged the system in the first place! As far as evacuating it real when you open the system to replace component, please take it to a place which can take the refrigerant out. Take it back to them for refill, they might credit you for your extraction.
As far as overcharging is concerned, don't be greedy. That way, you will never overcharge it.