It depends on the customs of where you are. I know in the US it is most certainly expected as tipped employees (including waiters and delivery drivers) make less than minimum wage and depend almost solely on their tips for their income.
I know here in Texas that servers make $2.13 an hour, when minimum wage is $7.25. Pizza delivery drivers make $4.25 an hour when delivering.
If you don't tip they can't pay the rent.