The only time I use skycaps is when our entire family of 6 is traveling and then, I view it as a tremendous convenience to be able to dump 5 or 6 bags and perhaps some golf clubs curbside to free my wife up to deal with four young children inside the terminal and to allow me to either park the car or return the rental. As this situation only occurs when we are on vacation, I am in a good mood and I try to value the convenience of the service rendered. For that, I usually tip $20--which I view as reasonable value.