Most waiters and waitresses are entirely dependent on tips. My girlfriend works as a waitress so has a good idea of customs, and she says that only Oromo and older Somalis don’t tip, but all other ethnicities and young Somalis do. Some tend to give a low note, for example 5 or 10 Birr, some just give whatever their change is, particularly Arabs, who will pay a 120 Birr bill or a 195 Birr bill with two 100 Birr notes and leave the change.
The focus on whether to tip should be based on the fact that waiters and waitresses get around 250-450 Birr a month for six 9-hour shifts a week. This is less than 14 Birr, 50p or 70 cents for a 9-hour shift, a third of which goes on their transport if they work evenings. So clearly they are utterly dependent on tips. In one of the main Piassa hotels waitresses start on 250 Birr a month, which after a year goes up to 300. One prestigious restaurant charging 80 Birr for a pizza offers 700 Birr a month, for four double-shifts a week, ie, 64 hours a week.
I notice too many tourists not tipping and I think they’d be just as happy leaving a tip if they felt it was appropriate, and it would make an enormous difference to the staff, many of whom are single mothers.