Only take leaves that drop with a slight tug. IF they are still firmly attached to the plant, even if partially damaged, the plant is still using them to scavenge the nutes contained in their tissues before sloughing them off. Read up on the definition of 'mobile nutrients'.

Don't take leaves off until they are ready to drop on their own, unless they are severely damaged by fungus or infested by a pest, and must be removed from the grow area and incinerated.