How should I use a static-shielding bag prior to opening, and why?
Antistatic precautions mean you should never move a device through a large potential difference. 

Once you and the ESD mat are connected to ground, they are at the same 0v potential. Take hold of the bag, this bring it and its contents to 0v. Now you can open the bag and handle the devices with anything that's also at 0v. This includes fingers, or conductive things you are holding. So metal tweezers, 'antistatic' pliers that have special conductive plastic handle covers, cotton gloves (which retain enough moisture to be slightly conductive) but NOT tools with insulating handle covers or plastic gloves.