There is always a question about whether to pay your tip with cash or credit card. Paying cash for tips will ensure that they get to the person they are intended for. If you add them onto the credit card charge, One of two things will occur. Often the establishment has a system for dividing them between all the staff at the end of the day/week in a pre-determined formula or unacceptably, not allocating them to staff.
Especially when good service is rendered you will want to ensure that the appropriate person gets the reward. On the other hand consider the dishwasher who does not interact with the public who may benefit by the “manager’s discretion” approach. An alternative is to add the standard tip to the credit card charge. Then give individuals who provide good service an extra tip in cash.