Originally Posted by
Ldnn1
Indeed it's always 'discretionary', but it's very rare for customers to have it taken off.
I saw it happen at a Ping Pong restaurant near the Festival Hall by a young couple several years ago. They were saying this line is optional isn't it and the waiter was clearly shocked someone was challenging it and asking for it to be taken off. He had to go get the manager as he didn't know how to do it!