I do not believe the frequency of people "stiffing" a server will be any higher on the DDP than it is without. If someone is a good, decent, and respectable person they will leave a tip; if they are of the opposite disposition, they would be hard pressed to leave a tip in any situation. If you receive poor service just because you are using the DDP, then your server is the one with the surly, less than respectable attitude. Use of the DDP is not going to turn the majority of people into non-tipping slobs, but servers with preconceived notions of DDP users could result in bad service which may earn them poor tips.