High quality nylon-tipped tweezers for handling watch batteries.
Why? - two reasons.
Firstly, if you use metal tweezers and accidentally touch both sides of the battery, you will "short" it. It will still work, but only for a a week or two, then the customer will complain.
Secondly, getting greasy finger marks on a battery shortens its life. If the battery is for a scale or a torch, this won't be noticeable, but if it's meant to last for two or three years in a watch and only last a few months...the customer will complain.