You should be using some sort of adhesive so the battery doesn't move around while it's installed in the phone.
Because if there is little to no adhesive you risk having the volume / power button ribbon / flex cables being cut from all the friction caused by the battery moving around. There is even a chance for the battery ribbon cable to be cut from this too.
Additionally if these phones came from clients or customers the chances are you will see them come back and complain about the battery moving because they can hear and feel the battery moving around.
You have a few options on what kind of adhesive you want to use.
The cheap quality one is just black double sided tape from eBay, next up is red tape (transparent adhesive) which you can also buy from eBay.
The best quality ones are the tesa tapes or 2x adhesive battery strips.
That's why I always come prepared with a decent amount of adhesive for battery or screen replacements without frame pre installed.
Edit: If you were asking about reusing old adhesive, it's highly recommended that you do not as they weaken a lot after being used once.