Finally heard back from my wife's nephew. He says his guys get about one $10-$20 tip a week, so it isn't the norm to get a tip, and he says it is almost always when a customer wants to keep the old appliance (like a fridge) and asks the delivery guys to set up the new one, and then move and setup the old one in a new location like the garage.
His region has 16 stores, only 2 still do their own deliveries. Independent guys (owner operators) get $59 a delivery plus mileage to cover the cost of the truck and fuel. 2 man crew, so no way to know what the helper gets paid, the the owner operator is earning at least $29.50 per delivery, more if he gives the helper less than half. They do about 12 deliveries a day. so the owner operator is making at least $44.25 an hour.
That puts them, in my book above the level of pay that I would tip.