I've done a good deal of manual labor and I don't think tipping is necessary. If for some reason the delivery folks do more than deliver a product (like say move some other furniture around a la Piobaire) then yes tip them.
Ha, I just told my wife today to tip the refrigerator delivery guys. She questioned me on it. Anyone who has done manual labor would probably agree with tipping. Even $5 makes these guys days.
I've done a good deal of manual labor and I don't think tipping is necessary. If for some reason the delivery folks do more than deliver a product (like say move some other furniture around a la Piobaire) then yes tip them.