When I move, I gotta pay for about 4 hours of work so it gets really expensive. Last time it cost me over $700 which was worth it my opinion. 3 guys showed up to do the work which was sufficient. When paying that much money, I like to ensure that it is quality and hassle free service. If there was an option to pay a little extra that ensures such a thing then I would do it, but there isn't.
Tipping is a mind game. One good reason to do it is for good service. Next time I plan to do something like get each of the 3 guys a $25 dollar gift card to a restaurant or something like that which is unique and I plan to give it to them before they start working. I understand why people might think that is nuts, but to me it is for the best. Also, giving them water is a must no matter how cheap you are.