Typically I tip $5 but that is because I delivered for Dominos for 2 yrs. Before I delivered, I probably rounded up to the nearest $5 which seems quite common here. When the total was $16 and change, nearly everyone would give a $20 and say keep the change. When Dominos raised prices and the same pizzas went to $17 and change the drivers typically got the same.
My best tips definitely came from Friday/Saturday night drinkers, lol. Once I got a $50 tip (for a party delivery of 10 pizzas) and another time I had a guy ask if the driver would pick up a pack of diapers on the way for an extra $20. Luckily I never ran across any real weirdos but I did have my share of kids (14 yrs or so) who liked to pay the exact amount with their allowance - all in change!