I went to a restaurant yesterday in LA (Take A Bao, Studio City....pretty tasty, fyi)...you ordered at the counter, they gave you your drinks and a magnetized number which you then put on a post at your table (self-seating). The bus boy would then bring your food.
I've been to other restaurants with service like this (getty museum in malibu, for example) and never know what and if I should tip. Standard 18-20 seems too much, but nothing seems wrong too. And just leaving a few bucks seems cheap.