Well, I haven't had curbside delivery, for furniture, so I'm not really sure how that differs. My only deliveries where I wouldn't tip are things like UPS/FedEx where they just drop it at the door. When I've had furniture or electronics delivered to my house, not setup just delivered, I've always tipped. Mind you, I don't go out to the truck to help unload the stuff, that's why I'm getting delivery. Consequently, as long as they don't break anything in the house, I always tip. It is hard work, and they do deserve a little something.
As Neuner mentioned, at least give their boss compliments to the service. That kind of stuff looks really good for the employees, and can go a long way.
Just a few thoughts...