Generally I try to be gracious, but it is all situational. If they use a company vehicle, I normally don't give a monetary tip, but I do offer them refreshments if they have to carry something into my house. If it's a pizza person driving their own vehicle, I tip money. If there is already a delivery charge for the service, they usually get a percentage or all of the delivery charge, but if they are driving their own car I feel sorry for them.
In my country employers are allowed to pay people in certain professions less than the minimum wage. These are the people who depend on our tips.