You are definitively right: Ciao is used to greet someone or to say goodbye in a informal way. Unfortunately Italian is getting more and more informal and people don't make any difference between formal and informal language, which is not good at all. I think you should use the proper Italian you are learning. Sarah

Those waiters were acting familiar to you, possibly for being close to your age, or you being younger than them, they would not just say it to a middle aged guy unless he was a regular. If you went to a "serious" restaurant, a waiter/maitre would have addressed you more formally. It's not as much a 'stranger' rule rather than a 'respect' rule. You can say ciao to a girl your age you meet, but not to an older woman you don't know well, or to a teacher, a cop, a boss. If you were a receptionist, you would be required to greet the clients, even as young as you, with a more formal 'buongiorno'.