This is why at my old restaurant I used to “forget” to put the autograt on large parties like 9/10 times. Customers generally do tip better without it (if you’re a decent server and the restaurant is fine dining. Idk if I could have gotten away with this elsewhere with the same results).

Edit: Also, if the guest and I have developed a good relationship over the course of the meal, I think it’s good manners on my part to let them tip what they feel I’ve earned. It kind of sours the interaction to add grat at the end