Like Tommy said, sometimes if they have a jar.
If it's a delivery at home - always.
No. Sometimes I don't even leave a tip when being served. I live in California; minimum wage here is $11 INCLUDING FOR SERVERS, and in a few years it will go up to $15. Tipping servers is to make up lost wages, but in California that's not a thing, so you don't need to tip here.
If you think that $11 an hour is a very good wage then perhaps you shouldn't tip. Don't expect them to go out of their way either though when you need an extra napkin or a refill on your lemonade.