Where the server brings the credit card slip to your table and picks it up after you complete it, s/he can see what you wrote. If management tries to stiff the server after that, at least s/he knows what's going on. It's his/her issue at that point, not mine.

Where you take your bill to a cashier, I always tell servers "I'm not stiffing you, your tip will be on the credit card slip." So far nobody has objected, though many have thanked me for letting them know. If they had concerns about not getting their tips I think some would have said something.

The above posts about management not giving tips to the wait staff sound more like individual cases, either restaurants in deep financial yogurt or crooks running the joint, than a general concern about credit cards not being a viable way to tip from the server's point of view.