If there is table or bar service you need to tip. If it's a big city, 20% is considered a decent tip. Bar tips are often 25%. You should tip delivery drivers too.
Yes the system is unfair.
No the system being unfair doesn't make you less of an asshole for undertipping or stiffing people.
HOWEVER.
You are under no obligation to tip people who aren't paid tipped wage (under minimum wage) or aren't delivering. When they bring the screen to you, press zero.